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From owner-pgsql-hackers@hub.org Mon Mar 22 18:43:41 1999
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Mon, 22 Mar 1999 18:27:15 -0500 (EST)
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Message-ID: <sqxh7H_00gNtAmTJ5Q@andrew.cmu.edu>
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Date: Mon, 22 Mar 1999 18:27:15 -0500 (EST)
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From: Erik Riedel <riedel+@CMU.EDU>
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To: pgsql-hackers@postgreSQL.org
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Subject: [HACKERS] optimizer and type question
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Sender: owner-pgsql-hackers@postgreSQL.org
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Precedence: bulk
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Status: RO
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[last week aggregation, this week, the optimizer]
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I have a somewhat general optimizer question/problem that I would like
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to get some input on - i.e. I'd like to know what is "supposed" to
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work here and what I should be expecting. Sadly, I think the patch
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for this is more involved than my last message.
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Using my favorite table these days:
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Table = lineitem
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+------------------------+----------------------------------+-------+
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| Field | Type | Length|
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+------------------------+----------------------------------+-------+
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| l_orderkey | int4 not null | 4 |
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| l_partkey | int4 not null | 4 |
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| l_suppkey | int4 not null | 4 |
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| l_linenumber | int4 not null | 4 |
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| l_quantity | float4 not null | 4 |
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| l_extendedprice | float4 not null | 4 |
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| l_discount | float4 not null | 4 |
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| l_tax | float4 not null | 4 |
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| l_returnflag | char() not null | 1 |
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| l_linestatus | char() not null | 1 |
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| l_shipdate | date | 4 |
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| l_commitdate | date | 4 |
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| l_receiptdate | date | 4 |
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| l_shipinstruct | char() not null | 25 |
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| l_shipmode | char() not null | 10 |
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| l_comment | char() not null | 44 |
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+------------------------+----------------------------------+-------+
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Index: lineitem_index_
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and the query:
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--
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-- Query 1
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--
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explain select l_returnflag, l_linestatus, sum(l_quantity) as sum_qty,
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sum(l_extendedprice) as sum_base_price,
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sum(l_extendedprice*(1-l_discount)) as sum_disc_price,
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sum(l_extendedprice*(1-l_discount)*(1+l_tax)) as sum_charge,
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avg(l_quantity) as avg_qty, avg(l_extendedprice) as avg_price,
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avg(l_discount) as avg_disc, count(*) as count_order
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from lineitem
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where l_shipdate <= '1998-09-02'::date
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group by l_returnflag, l_linestatus
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order by l_returnflag, l_linestatus;
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note that I have eliminated the date calculation in my query of last
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week and manually replaced it with a constant (since this wasn't
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happening automatically - but let's not worry about that for now).
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And this is only an explain, we care about the optimizer. So we get:
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Sort (cost=34467.88 size=0 width=0)
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-> Aggregate (cost=34467.88 size=0 width=0)
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-> Group (cost=34467.88 size=0 width=0)
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-> Sort (cost=34467.88 size=0 width=0)
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-> Seq Scan on lineitem (cost=34467.88 size=200191 width=44)
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so let's think about the selectivity that is being chosen for the
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seq scan (the where l_shipdate <= '1998-09-02').
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Turns out the optimizer is choosing "33%", even though the real answer
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is somewhere in 90+% (that's how the query is designed). So, why does
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it do that?
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Turns out that selectivity in this case is determined via
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plancat::restriction_selectivity() which calls into functionOID = 103
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(intltsel) for operatorOID = 1096 (date "<=") on relation OID = 18663
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(my lineitem).
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This all follows because of the description of 1096 (date "<=") in
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pg_operator. Looking at local1_template1.bki.source near line 1754
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shows:
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insert OID = 1096 ( "<=" PGUID 0 <...> date_le intltsel intltjoinsel )
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where we see that indeed, it thinks "intltsel" is the right function
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to use for "oprrest" in the case of dates.
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Question 1 - is intltsel the right thing for selectivity on dates?
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Hope someone is still with me.
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So now we're running selfuncs::intltsel() where we make a further call
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to selfuncs::gethilokey(). The job of gethilokey is to determine the
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min and max values of a particular attribute in the table, which will
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then be used with the constant in my where clause to estimate the
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selectivity. It is going to search the pg_statistic relation with
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three key values:
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Anum_pg_statistic_starelid 18663 (lineitem)
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Anum_pg_statistic_staattnum 11 (l_shipdate)
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Anum_pg_statistic_staop 1096 (date "<=")
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this finds no tuples in pg_statistic. Why is that? The only nearby
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tuple in pg_statistic is:
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starelid|staattnum|staop|stalokey |stahikey
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--------+---------+-----+----------------+----------------
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18663| 11| 0|01-02-1992 |12-01-1998
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and the reason the query doesn't match anything? Because 1096 != 0.
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But why is it 0 in pg_statistic? Statistics are determined near line
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1844 in vacuum.c (assuming a 'vacuum analyze' run at some point)
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i = 0;
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values[i++] = (Datum) relid; /* 1 */
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values[i++] = (Datum) attp->attnum; /* 2 */
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====> values[i++] = (Datum) InvalidOid; /* 3 */
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fmgr_info(stats->outfunc, &out_function);
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out_string = <...min...>
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values[i++] = (Datum) fmgr(F_TEXTIN, out_string);
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pfree(out_string);
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out_string = <...max...>
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values[i++] = (Datum) fmgr(F_TEXTIN, out_string);
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pfree(out_string);
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stup = heap_formtuple(sd->rd_att, values, nulls);
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the "offending" line is setting the staop to InvalidOid (i.e. 0).
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Question 2 - is this right? Is the intent for 0 to serve as a
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"wildcard", or should it be inserting an entry for each operation
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individually?
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In the case of "wildcard" then gethilokey() should allow a match for
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Anum_pg_statistic_staop 0
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instead of requiring the more restrictive 1096. In the current code,
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what happens next is gethilokey() returns "not found" and intltsel()
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returns the default 1/3 which I see in the resultant query plan (size
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= 200191 is 1/3 of the number of lineitem tuples).
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Question 3 - is there any inherent reason it couldn't get this right?
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The statistic is in the table 1992 to 1998, so the '1998-09-02' date
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should be 90-some% selectivity, a much better guess than 33%.
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Doesn't make a difference for this particular query, of course,
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because the seq scan must proceed anyhow, but it could easily affect
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other queries where selectivities matter (and it affects the
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modifications I am trying to test in the optimizer to be "smarter"
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about selectivities - my overall context is to understand/improve the
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behavior that the underlying storage system sees from queries like this).
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OK, so let's say we treat 0 as a "wildcard" and stop checking for
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1096. Not we let gethilokey() return the two dates from the statistic
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table. The immediate next thing that intltsel() does, near lines 122
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in selfuncs.c is call atol() on the strings from gethilokey(). And
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guess what it comes up with?
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low = 1
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high = 12
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because it calls atol() on '01-02-1992' and '12-01-1998'. This
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clearly isn't right, it should get some large integer that includes
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the year and day in the result. Then it should compare reasonably
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with my constant from the where clause and give a decent selectivity
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value. This leads to a re-visit of Question 1.
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Question 4 - should date "<=" use a dateltsel() function instead of
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intltsel() as oprrest?
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If anyone is still with me, could you tell me if this makes sense, or
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if there is some other location where the appropriate type conversion
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could take place so that intltsel() gets something reasonable when it
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does the atol() calls?
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Could someone also give me a sense for how far out-of-whack the whole
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current selectivity-handling structure is? It seems that most of the
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operators in pg_operator actually use intltsel() and would have
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type-specific problems like that described. Or is the problem in the
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way attribute values are stored in pg_statistic by vacuum analyze? Or
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is there another layer where type conversion belongs?
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Phew. Enough typing, hope someone can follow this and address at
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least some of the questions.
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Thanks.
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Erik Riedel
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Carnegie Mellon University
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www.cs.cmu.edu/~riedel
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From owner-pgsql-hackers@hub.org Mon Mar 22 20:31:11 1999
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Received: from renoir.op.net (root@renoir.op.net [209.152.193.4])
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Mon, 22 Mar 1999 20:12:43 -0500 (EST)
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To: Erik Riedel <riedel+@CMU.EDU>
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cc: pgsql-hackers@postgreSQL.org
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Subject: Re: [HACKERS] optimizer and type question
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In-reply-to: Your message of Mon, 22 Mar 1999 18:27:15 -0500 (EST)
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<sqxh7H_00gNtAmTJ5Q@andrew.cmu.edu>
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Date: Mon, 22 Mar 1999 20:12:43 -0500
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Message-ID: <23292.922151563@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Sender: owner-pgsql-hackers@postgreSQL.org
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Precedence: bulk
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Status: ROr
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Erik Riedel <riedel+@CMU.EDU> writes:
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> [ optimizer doesn't find relevant pg_statistic entry ]
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It's clearly a bug that the selectivity code is not finding this tuple.
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If your analysis is correct, then selectivity estimation has *never*
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worked properly, or at least not in recent memory :-(. Yipes.
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Bruce and I found a bunch of other problems in the optimizer recently,
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so it doesn't faze me to assume that this is broken too.
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> the "offending" line is setting the staop to InvalidOid (i.e. 0).
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> Question 2 - is this right? Is the intent for 0 to serve as a
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> "wildcard",
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My thought is that what the staop column ought to be is the OID of the
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comparison function that was used to determine the sort order of the
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column. Without a sort op the lowest and highest keys in the column are
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not well defined, so it makes no sense to assert "these are the lowest
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and highest values" without providing the sort op that determined that.
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(For sufficiently complex data types one could reasonably have multiple
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ordering operators. A crude example is sorting on "circumference" and
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"area" for polygons.) But typically the sort op will be the "<"
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operator for the column data type.
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So, the vacuum code is definitely broken --- it's not storing the sort
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op that it used. The code in gethilokey might be broken too, depending
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on how it is producing the operator it's trying to match against the
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tuple. For example, if the actual operator in the query is any of
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< <= > >= on int4, then int4lt ought to be used to probe the pg_statistic
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table. I'm not sure if we have adequate info in pg_operator or pg_type
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to let the optimizer code determine the right thing to probe with :-(
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> The immediate next thing that intltsel() does, near lines 122
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> in selfuncs.c is call atol() on the strings from gethilokey(). And
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> guess what it comes up with?
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> low = 1
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> high = 12
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> because it calls atol() on '01-02-1992' and '12-01-1998'. This
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> clearly isn't right, it should get some large integer that includes
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> the year and day in the result. Then it should compare reasonably
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> with my constant from the where clause and give a decent selectivity
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> value. This leads to a re-visit of Question 1.
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> Question 4 - should date "<=" use a dateltsel() function instead of
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> intltsel() as oprrest?
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This is clearly busted as well. I'm not sure that creating dateltsel()
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is the right fix, however, because if you go down that path then every
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single datatype needs its own selectivity function; that's more than we
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need.
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What we really want here is to be able to map datatype values into
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some sort of numeric range so that we can compute what fraction of the
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low-key-to-high-key range is on each side of the probe value (the
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constant taken from the query). This general concept will apply to
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many scalar types, so what we want is a type-specific mapping function
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and a less-specific fraction-computing-function. Offhand I'd say that
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we want intltsel() and floatltsel(), plus conversion routines that can
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produce either int4 or float8 from a data type as seems appropriate.
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Anything that couldn't map to one or the other would have to supply its
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own selectivity function.
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> Or is the problem in the
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> way attribute values are stored in pg_statistic by vacuum analyze?
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Looks like it converts the low and high values to text and stores them
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that way. Ugly as can be :-( but I'm not sure there is a good
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alternative. We have no "wild card" column type AFAIK, which is what
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these columns of pg_statistic would have to be to allow storage of
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unconverted min and max values.
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I think you've found a can of worms here. Congratulations ;-)
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regards, tom lane
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From owner-pgsql-hackers@hub.org Mon Mar 22 23:31:00 1999
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Received: from mms.4.60.Jun.27.1996.03.02.53.sun4.51.EzMail.2.0.CUILIB.3.45.SNAP.NOT.LINKED.hazy.adsl.net.cmu.edu.sun4m.54
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via MS.5.6.hazy.adsl.net.cmu.edu.sun4_51;
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Mon, 22 Mar 1999 23:14:55 -0500 (EST)
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Message-ID: <4qxlJ0200anI01hK40@andrew.cmu.edu>
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Date: Mon, 22 Mar 1999 23:14:55 -0500 (EST)
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From: Erik Riedel <riedel+@CMU.EDU>
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To: Tom Lane <tgl@sss.pgh.pa.us>
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Subject: Re: [HACKERS] optimizer and type question
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Cc: pgsql-hackers@postgreSQL.org
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In-Reply-To: <23292.922151563@sss.pgh.pa.us>
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References: <23292.922151563@sss.pgh.pa.us>
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Sender: owner-pgsql-hackers@postgreSQL.org
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Precedence: bulk
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Status: ROr
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OK, building on your high-level explanation, I am attaching a patch that
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attempts to do something "better" than the current code. Note that I
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have only tested this with the date type and my particular query. I
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haven't run it through the regression, so consider it "proof of concept"
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at best. Although hopefully it will serve my purposes.
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> My thought is that what the staop column ought to be is the OID of the
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> comparison function that was used to determine the sort order of the
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> column. Without a sort op the lowest and highest keys in the column are
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> not well defined, so it makes no sense to assert "these are the lowest
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> and highest values" without providing the sort op that determined that.
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>
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> (For sufficiently complex data types one could reasonably have multiple
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> ordering operators. A crude example is sorting on "circumference" and
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> "area" for polygons.) But typically the sort op will be the "<"
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> operator for the column data type.
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>
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I changed vacuum.c to do exactly that. oid of the lt sort op.
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> So, the vacuum code is definitely broken --- it's not storing the sort
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> op that it used. The code in gethilokey might be broken too, depending
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> on how it is producing the operator it's trying to match against the
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> tuple. For example, if the actual operator in the query is any of
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> < <= > >= on int4, then int4lt ought to be used to probe the pg_statistic
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> table. I'm not sure if we have adequate info in pg_operator or pg_type
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> to let the optimizer code determine the right thing to probe with :-(
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>
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This indeed seems like a bigger problem. I thought about somehow using
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type-matching from the sort op and the actual operator in the query - if
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both the left and right type match, then consider them the same for
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purposes of this probe. That seemed complicated, so I punted in my
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example - it just does the search with relid and attnum and assumes that
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only returns one tuple. This works in my case (maybe in all cases,
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because of the way vacuum is currently written - ?).
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> What we really want here is to be able to map datatype values into
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> some sort of numeric range so that we can compute what fraction of the
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> low-key-to-high-key range is on each side of the probe value (the
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> constant taken from the query). This general concept will apply to
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> many scalar types, so what we want is a type-specific mapping function
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> and a less-specific fraction-computing-function. Offhand I'd say that
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> we want intltsel() and floatltsel(), plus conversion routines that can
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> produce either int4 or float8 from a data type as seems appropriate.
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> Anything that couldn't map to one or the other would have to supply its
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> own selectivity function.
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>
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This is what my example then does. Uses the stored sort op to get the
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type and then uses typinput to convert from the string to an int4.
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Then puts the int4 back into string format because that's what everyone
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was expecting.
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It seems to work for my particular query. I now get:
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(selfuncs) gethilokey() obj 18663 attr 11 opid 1096 (ignored)
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(selfuncs) gethilokey() found op 1087 in pg_proc
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(selfuncs) gethilokey() found type 1082 in pg_type
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(selfuncs) gethilokey() going to use 1084 to convert type 1082
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(selfuncs) gethilokey() have low -2921 high -396
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(selfuncs) intltsel() high -396 low -2921 val -486
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(plancat) restriction_selectivity() for func 103 op 1096 rel 18663 attr
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11 const -486 flag 3 returns 0.964356
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NOTICE: QUERY PLAN:
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Sort (cost=34467.88 size=0 width=0)
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-> Aggregate (cost=34467.88 size=0 width=0)
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-> Group (cost=34467.88 size=0 width=0)
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-> Sort (cost=34467.88 size=0 width=0)
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-> Seq Scan on lineitem (cost=34467.88 size=579166 width=44)
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including my printfs, which exist in the patch as well.
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Selectivity is now the expected 96% and the size estimate for the seq
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scan is much closer to correct.
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Again, not tested with anything besides date, so caveat not-tested.
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Hope this helps.
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Erik
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----------------------[optimizer_fix.sh]------------------------
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#! /bin/sh
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# This is a shell archive, meaning:
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# 1. Remove everything above the #! /bin/sh line.
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# 2. Save the resulting text in a file.
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# 3. Execute the file with /bin/sh (not csh) to create:
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# selfuncs.c.diff
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# vacuum.c.diff
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# This archive created: Mon Mar 22 22:58:14 1999
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export PATH; PATH=/bin:/usr/bin:$PATH
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if test -f 'selfuncs.c.diff'
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then
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echo shar: "will not over-write existing file 'selfuncs.c.diff'"
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else
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cat << \SHAR_EOF > 'selfuncs.c.diff'
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***
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/afs/ece.cmu.edu/project/lcs/lcs-004/er1p/postgres/611/src/backend/utils/adt
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/selfuncs.c Thu Mar 11 23:59:35 1999
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---
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/afs/ece.cmu.edu/project/lcs/lcs-004/er1p/postgres/615/src/backend/utils/adt
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/selfuncs.c Mon Mar 22 22:57:25 1999
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***************
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*** 32,37 ****
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--- 32,40 ----
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#include "utils/lsyscache.h" /* for get_oprrest() */
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#include "catalog/pg_statistic.h"
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+ #include "catalog/pg_proc.h" /* for Form_pg_proc */
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+ #include "catalog/pg_type.h" /* for Form_pg_type */
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+
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/* N is not a valid var/constant or relation id */
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#define NONVALUE(N) ((N) == -1)
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***************
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*** 103,110 ****
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bottom;
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result = (float64) palloc(sizeof(float64data));
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! if (NONVALUE(attno) || NONVALUE(relid))
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*result = 1.0 / 3;
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else
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{
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/* XXX val = atol(value); */
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--- 106,114 ----
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bottom;
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result = (float64) palloc(sizeof(float64data));
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! if (NONVALUE(attno) || NONVALUE(relid)) {
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*result = 1.0 / 3;
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+ }
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else
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{
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/* XXX val = atol(value); */
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***************
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*** 117,130 ****
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}
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high = atol(highchar);
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low = atol(lowchar);
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if ((flag & SEL_RIGHT && val < low) ||
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(!(flag & SEL_RIGHT) && val > high))
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{
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float32data nvals;
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nvals = getattdispersion(relid, (int) attno);
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! if (nvals == 0)
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*result = 1.0 / 3.0;
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else
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{
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*result = 3.0 * (float64data) nvals;
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--- 121,136 ----
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}
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high = atol(highchar);
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low = atol(lowchar);
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+ printf("(selfuncs) intltsel() high %d low %d val %d\n",high,low,val);
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if ((flag & SEL_RIGHT && val < low) ||
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(!(flag & SEL_RIGHT) && val > high))
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{
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float32data nvals;
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nvals = getattdispersion(relid, (int) attno);
|
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! if (nvals == 0) {
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*result = 1.0 / 3.0;
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+ }
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else
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{
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*result = 3.0 * (float64data) nvals;
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***************
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*** 336,341 ****
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--- 342,353 ----
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{
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Relation rel;
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HeapScanDesc scan;
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+ /* this assumes there is only one row in the statistics table for any
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particular */
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+ /* relid, attnum pair - could be more complicated if staop is also
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used. */
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+ /* at the moment, if there are multiple rows, this code ends up
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picking the */
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+ /* "first" one
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- er1p */
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+ /* the actual "ignoring" is done in the call to heap_beginscan()
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below, where */
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+ /* we only mention 2 of the 3 keys in this array
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- er1p */
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static ScanKeyData key[3] = {
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{0, Anum_pg_statistic_starelid, F_OIDEQ, {0, 0, F_OIDEQ}},
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{0, Anum_pg_statistic_staattnum, F_INT2EQ, {0, 0, F_INT2EQ}},
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***************
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*** 344,355 ****
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|
bool isnull;
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|
HeapTuple tuple;
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|
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rel = heap_openr(StatisticRelationName);
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|
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key[0].sk_argument = ObjectIdGetDatum(relid);
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key[1].sk_argument = Int16GetDatum((int16) attnum);
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key[2].sk_argument = ObjectIdGetDatum(opid);
|
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! scan = heap_beginscan(rel, 0, SnapshotNow, 3, key);
|
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tuple = heap_getnext(scan, 0);
|
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if (!HeapTupleIsValid(tuple))
|
|
{
|
|
--- 356,377 ----
|
|
bool isnull;
|
|
HeapTuple tuple;
|
|
|
|
+ HeapTuple tup;
|
|
+ Form_pg_proc proc;
|
|
+ Form_pg_type typ;
|
|
+ Oid which_op;
|
|
+ Oid which_type;
|
|
+ int32 low_value;
|
|
+ int32 high_value;
|
|
+
|
|
rel = heap_openr(StatisticRelationName);
|
|
|
|
key[0].sk_argument = ObjectIdGetDatum(relid);
|
|
key[1].sk_argument = Int16GetDatum((int16) attnum);
|
|
key[2].sk_argument = ObjectIdGetDatum(opid);
|
|
! printf("(selfuncs) gethilokey() obj %d attr %d opid %d (ignored)\n",
|
|
! key[0].sk_argument,key[1].sk_argument,key[2].sk_argument);
|
|
! scan = heap_beginscan(rel, 0, SnapshotNow, 2, key);
|
|
tuple = heap_getnext(scan, 0);
|
|
if (!HeapTupleIsValid(tuple))
|
|
{
|
|
***************
|
|
*** 376,383 ****
|
|
--- 398,461 ----
|
|
&isnull));
|
|
if (isnull)
|
|
elog(DEBUG, "gethilokey: low key is null");
|
|
+
|
|
heap_endscan(scan);
|
|
heap_close(rel);
|
|
+
|
|
+ /* now we deal with type conversion issues
|
|
*/
|
|
+ /* when intltsel() calls this routine (who knows what other callers
|
|
might do) */
|
|
+ /* it assumes that it can call atol() on the strings and then use
|
|
integer */
|
|
+ /* comparison from there. what we are going to do here, then, is try
|
|
to use */
|
|
+ /* the type information from Anum_pg_statistic_staop to convert the
|
|
high */
|
|
+ /* and low values
|
|
- er1p */
|
|
+
|
|
+ /* WARNING: this code has only been tested with the date type and has
|
|
NOT */
|
|
+ /* been regression tested. consider it "sample" code of what might
|
|
be the */
|
|
+ /* right kind of thing to do
|
|
- er1p */
|
|
+
|
|
+ /* get the 'op' from pg_statistic and look it up in pg_proc */
|
|
+ which_op = heap_getattr(tuple,
|
|
+ Anum_pg_statistic_staop,
|
|
+ RelationGetDescr(rel),
|
|
+ &isnull);
|
|
+ if (InvalidOid == which_op) {
|
|
+ /* ignore all this stuff, try conversion only if we have a valid staop */
|
|
+ /* note that there is an accompanying change to 'vacuum analyze' that */
|
|
+ /* gets this set to something useful. */
|
|
+ } else {
|
|
+ /* staop looks valid, so let's see what we can do about conversion */
|
|
+ tup = SearchSysCacheTuple(PROOID, ObjectIdGetDatum(which_op), 0, 0, 0);
|
|
+ if (!HeapTupleIsValid(tup)) {
|
|
+ elog(ERROR, "selfuncs: unable to find op in pg_proc %d", which_op);
|
|
+ }
|
|
+ printf("(selfuncs) gethilokey() found op %d in pg_proc\n",which_op);
|
|
+
|
|
+ /* use that to determine the type of stahikey and stalokey via pg_type */
|
|
+ proc = (Form_pg_proc) GETSTRUCT(tup);
|
|
+ which_type = proc->proargtypes[0]; /* XXX - use left and right
|
|
separately? */
|
|
+ tup = SearchSysCacheTuple(TYPOID, ObjectIdGetDatum(which_type), 0, 0, 0);
|
|
+ if (!HeapTupleIsValid(tup)) {
|
|
+ elog(ERROR, "selfuncs: unable to find type in pg_type %d", which_type);
|
|
+ }
|
|
+ printf("(selfuncs) gethilokey() found type %d in pg_type\n",which_type);
|
|
+
|
|
+ /* and use that type to get the conversion function to int4 */
|
|
+ typ = (Form_pg_type) GETSTRUCT(tup);
|
|
+ printf("(selfuncs) gethilokey() going to use %d to convert type
|
|
%d\n",typ->typinput,which_type);
|
|
+
|
|
+ /* and convert the low and high strings */
|
|
+ low_value = (int32) fmgr(typ->typinput, *low, -1);
|
|
+ high_value = (int32) fmgr(typ->typinput, *high, -1);
|
|
+ printf("(selfuncs) gethilokey() have low %d high
|
|
%d\n",low_value,high_value);
|
|
+
|
|
+ /* now we have int4's, which we put back into strings because
|
|
that's what out */
|
|
+ /* callers (intltsel() at least) expect
|
|
- er1p */
|
|
+ pfree(*low); pfree(*high); /* let's not leak the old strings */
|
|
+ *low = int4out(low_value);
|
|
+ *high = int4out(high_value);
|
|
+
|
|
+ /* XXX - this probably leaks the two tups we got from
|
|
SearchSysCacheTuple() - er1p */
|
|
+ }
|
|
}
|
|
|
|
float64
|
|
SHAR_EOF
|
|
fi
|
|
if test -f 'vacuum.c.diff'
|
|
then
|
|
echo shar: "will not over-write existing file 'vacuum.c.diff'"
|
|
else
|
|
cat << \SHAR_EOF > 'vacuum.c.diff'
|
|
***
|
|
/afs/ece.cmu.edu/project/lcs/lcs-004/er1p/postgres/611/src/backend/commands/
|
|
vacuum.c Thu Mar 11 23:59:09 1999
|
|
---
|
|
/afs/ece.cmu.edu/project/lcs/lcs-004/er1p/postgres/615/src/backend/commands/
|
|
vacuum.c Mon Mar 22 21:23:15 1999
|
|
***************
|
|
*** 1842,1848 ****
|
|
i = 0;
|
|
values[i++] = (Datum) relid; /* 1 */
|
|
values[i++] = (Datum) attp->attnum; /* 2 */
|
|
! values[i++] = (Datum) InvalidOid; /* 3 */
|
|
fmgr_info(stats->outfunc, &out_function);
|
|
out_string = (*fmgr_faddr(&out_function)) (stats->min,
|
|
stats->attr->atttypid);
|
|
values[i++] = (Datum) fmgr(F_TEXTIN, out_string);
|
|
--- 1842,1848 ----
|
|
i = 0;
|
|
values[i++] = (Datum) relid; /* 1 */
|
|
values[i++] = (Datum) attp->attnum; /* 2 */
|
|
! values[i++] = (Datum) stats->f_cmplt.fn_oid; /* 3 */ /* get the
|
|
'<' oid, instead of 'invalid' - er1p */
|
|
fmgr_info(stats->outfunc, &out_function);
|
|
out_string = (*fmgr_faddr(&out_function)) (stats->min,
|
|
stats->attr->atttypid);
|
|
values[i++] = (Datum) fmgr(F_TEXTIN, out_string);
|
|
SHAR_EOF
|
|
fi
|
|
exit 0
|
|
# End of shell archive
|
|
|
|
|
|
|
|
From owner-pgsql-hackers@hub.org Tue Mar 23 12:31:05 1999
|
|
Received: from renoir.op.net (root@renoir.op.net [209.152.193.4])
|
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by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id MAA17491
|
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for <maillist@candle.pha.pa.us>; Tue, 23 Mar 1999 12:31:04 -0500 (EST)
|
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Received: from hub.org (majordom@hub.org [209.47.145.100]) by renoir.op.net (o1/$ Revision: 1.18 $) with ESMTP id MAA08839 for <maillist@candle.pha.pa.us>; Tue, 23 Mar 1999 12:08:14 -0500 (EST)
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Received: from localhost (majordom@localhost)
|
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by hub.org (8.9.2/8.9.1) with SMTP id MAA93649;
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Tue, 23 Mar 1999 12:04:57 -0500 (EST)
|
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(envelope-from owner-pgsql-hackers@hub.org)
|
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Received: (from majordom@localhost)
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by hub.org (8.9.2/8.9.1) id MAA93355
|
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for pgsql-hackers-outgoing; Tue, 23 Mar 1999 12:02:55 -0500 (EST)
|
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(envelope-from owner-pgsql-hackers@postgreSQL.org)
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for <pgsql-hackers@postgreSQL.org>; Tue, 23 Mar 1999 12:02:43 -0500 (EST)
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Tue, 23 Mar 1999 12:01:57 -0500 (EST)
|
|
To: Erik Riedel <riedel+@CMU.EDU>
|
|
cc: pgsql-hackers@postgreSQL.org
|
|
Subject: Re: [HACKERS] optimizer and type question
|
|
In-reply-to: Your message of Mon, 22 Mar 1999 23:14:55 -0500 (EST)
|
|
<4qxlJ0200anI01hK40@andrew.cmu.edu>
|
|
Date: Tue, 23 Mar 1999 12:01:57 -0500
|
|
Message-ID: <24453.922208517@sss.pgh.pa.us>
|
|
From: Tom Lane <tgl@sss.pgh.pa.us>
|
|
Sender: owner-pgsql-hackers@postgreSQL.org
|
|
Precedence: bulk
|
|
Status: RO
|
|
|
|
Erik Riedel <riedel+@CMU.EDU> writes:
|
|
> OK, building on your high-level explanation, I am attaching a patch that
|
|
> attempts to do something "better" than the current code. Note that I
|
|
> have only tested this with the date type and my particular query.
|
|
|
|
Glad to see you working on this. I don't like the details of your
|
|
patch too much though ;-). Here are some suggestions for making it
|
|
better.
|
|
|
|
1. I think just removing staop from the lookup in gethilokey is OK for
|
|
now, though I'm dubious about Bruce's thought that we could delete that
|
|
field entirely. As you observe, vacuum will not currently put more
|
|
than one tuple for a column into pg_statistic, so we can just do the
|
|
lookup with relid and attno and leave it at that. But I think we ought
|
|
to leave the field there, with the idea that vacuum might someday
|
|
compute more than one statistic for a data column. Fixing vacuum to
|
|
put its sort op into the field is a good idea in the meantime.
|
|
|
|
2. The type conversion you're doing in gethilokey is a mess; I think
|
|
what you ought to make it do is simply the inbound conversion of the
|
|
string from pg_statistic into the internal representation for the
|
|
column's datatype, and return that value as a Datum. It also needs
|
|
a cleaner success/failure return convention --- this business with
|
|
"n" return is ridiculously type-specific. Also, the best and easiest
|
|
way to find the type to convert to is to look up the column type in
|
|
the info for the given relid, not search pg_proc with the staop value.
|
|
(I'm not sure that will even work, since there are pg_proc entries
|
|
with wildcard argument types.)
|
|
|
|
3. The atol() calls currently found in intltsel are a type-specific
|
|
cheat on what is conceptually a two-step process:
|
|
* Convert the string stored in pg_statistic back to the internal
|
|
form for the column data type.
|
|
* Generate a numeric representation of the data value that can be
|
|
used as an estimate of the range of values in the table.
|
|
The second step is trivial for integers, which may obscure the fact
|
|
that there are two steps involved, but nonetheless there are. If
|
|
you think about applying selectivity logic to strings, say, it
|
|
becomes clear that the second step is a necessary component of the
|
|
process. Furthermore, the second step must also be applied to the
|
|
probe value that's being passed into the selectivity operator.
|
|
(The probe value is already in internal form, of course; but it is
|
|
not necessarily in a useful numeric form.)
|
|
|
|
We can do the first of these steps by applying the appropriate "XXXin"
|
|
conversion function for the column data type, as you have done. The
|
|
interesting question is how to do the second one. A really clean
|
|
solution would require adding a column to pg_type that points to a
|
|
function that will do the appropriate conversion. I'd be inclined to
|
|
make all of these functions return "double" (float8) and just have one
|
|
top-level selectivity routine for all data types that can use
|
|
range-based selectivity logic.
|
|
|
|
We could probably hack something together that would not use an explicit
|
|
conversion function for each data type, but instead would rely on
|
|
type-specific assumptions inside the selectivity routines. We'd need many
|
|
more selectivity routines though (at least one for each of int, float4,
|
|
float8, and text data types) so I'm not sure we'd really save any work
|
|
compared to doing it right.
|
|
|
|
BTW, now that I look at this issue it's real clear that the selectivity
|
|
entries in pg_operator are horribly broken. The intltsel/intgtsel
|
|
selectivity routines are currently applied to 32 distinct data types:
|
|
|
|
regression=> select distinct typname,oprleft from pg_operator, pg_type
|
|
regression-> where pg_type.oid = oprleft
|
|
regression-> and oprrest in (103,104);
|
|
typname |oprleft
|
|
---------+-------
|
|
_aclitem | 1034
|
|
abstime | 702
|
|
bool | 16
|
|
box | 603
|
|
bpchar | 1042
|
|
char | 18
|
|
cidr | 650
|
|
circle | 718
|
|
date | 1082
|
|
datetime | 1184
|
|
float4 | 700
|
|
float8 | 701
|
|
inet | 869
|
|
int2 | 21
|
|
int4 | 23
|
|
int8 | 20
|
|
line | 628
|
|
lseg | 601
|
|
macaddr | 829
|
|
money | 790
|
|
name | 19
|
|
numeric | 1700
|
|
oid | 26
|
|
oid8 | 30
|
|
path | 602
|
|
point | 600
|
|
polygon | 604
|
|
text | 25
|
|
time | 1083
|
|
timespan | 1186
|
|
timestamp| 1296
|
|
varchar | 1043
|
|
(32 rows)
|
|
|
|
many of which are very obviously not compatible with integer for *any*
|
|
purpose. It looks to me like a lot of data types were added to
|
|
pg_operator just by copy-and-paste, without paying attention to whether
|
|
the selectivity routines were actually correct for the data type.
|
|
|
|
As the code stands today, the bogus entries don't matter because
|
|
gethilokey always fails, so we always get 1/3 as the selectivity
|
|
estimate for any comparison operator (except = and != of course).
|
|
I had actually noticed that fact and assumed that it was supposed
|
|
to work that way :-(. But, clearly, there is code in here that
|
|
is *trying* to be smarter.
|
|
|
|
As soon as we fix gethilokey so that it can succeed, we will start
|
|
getting essentially-random selectivity estimates for those data types
|
|
that aren't actually binary-compatible with integer. That will not do;
|
|
we have to do something about the issue.
|
|
|
|
regards, tom lane
|
|
|
|
|
|
From tgl@sss.pgh.pa.us Tue Mar 23 12:31:02 1999
|
|
Received: from renoir.op.net (root@renoir.op.net [209.152.193.4])
|
|
by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id MAA17484
|
|
for <maillist@candle.pha.pa.us>; Tue, 23 Mar 1999 12:31:01 -0500 (EST)
|
|
Received: from sss.sss.pgh.pa.us (sss.pgh.pa.us [206.210.65.6]) by renoir.op.net (o1/$ Revision: 1.18 $) with ESMTP id MAA09042 for <maillist@candle.pha.pa.us>; Tue, 23 Mar 1999 12:10:55 -0500 (EST)
|
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|
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by sss.sss.pgh.pa.us (8.9.1/8.9.1) with ESMTP id MAA24474;
|
|
Tue, 23 Mar 1999 12:09:52 -0500 (EST)
|
|
To: Bruce Momjian <maillist@candle.pha.pa.us>
|
|
cc: riedel+@CMU.EDU, pgsql-hackers@postgreSQL.org
|
|
Subject: Re: [HACKERS] optimizer and type question
|
|
In-reply-to: Your message of Mon, 22 Mar 1999 21:25:45 -0500 (EST)
|
|
<199903230225.VAA01641@candle.pha.pa.us>
|
|
Date: Tue, 23 Mar 1999 12:09:52 -0500
|
|
Message-ID: <24471.922208992@sss.pgh.pa.us>
|
|
From: Tom Lane <tgl@sss.pgh.pa.us>
|
|
Status: RO
|
|
|
|
Bruce Momjian <maillist@candle.pha.pa.us> writes:
|
|
> What we really need is some way to determine how far the requested value
|
|
> is from the min/max values. With int, we just do (val-min)/(max-min).
|
|
> That works, but how do we do that for types that don't support division.
|
|
> Strings come to mind in this case.
|
|
|
|
What I'm envisioning is that we still apply the (val-min)/(max-min)
|
|
logic, but apply it to numeric values that are produced in a
|
|
type-dependent way.
|
|
|
|
For ints and floats the conversion is trivial, of course.
|
|
|
|
For strings, the first thing that comes to mind is to return 0 for a
|
|
null string and the value of the first byte for a non-null string.
|
|
This would give you one-part-in-256 selectivity which is plenty good
|
|
enough for what the selectivity code needs to do. (Actually, it's
|
|
only that good if the strings' first bytes are pretty well spread out.
|
|
If you have a table containing English words, for example, you might
|
|
only get about one part in 26 this way, since the first bytes will
|
|
probably only run from A to Z. Might be better to use the first two
|
|
characters of the string to compute the selectivity representation.)
|
|
|
|
In general, you can apply this logic as long as you can come up with
|
|
some numerical approximation to the data type's sorting order. It
|
|
doesn't have to be exact.
|
|
|
|
regards, tom lane
|
|
|
|
From owner-pgsql-hackers@hub.org Tue Mar 23 12:31:03 1999
|
|
Received: from renoir.op.net (root@renoir.op.net [209.152.193.4])
|
|
by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id MAA17488
|
|
for <maillist@candle.pha.pa.us>; Tue, 23 Mar 1999 12:31:02 -0500 (EST)
|
|
Received: from hub.org (majordom@hub.org [209.47.145.100]) by renoir.op.net (o1/$ Revision: 1.18 $) with ESMTP id MAA09987 for <maillist@candle.pha.pa.us>; Tue, 23 Mar 1999 12:21:34 -0500 (EST)
|
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|
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|
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|
|
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|
|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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Tue, 23 Mar 1999 12:09:52 -0500 (EST)
|
|
To: Bruce Momjian <maillist@candle.pha.pa.us>
|
|
cc: riedel+@CMU.EDU, pgsql-hackers@postgreSQL.org
|
|
Subject: Re: [HACKERS] optimizer and type question
|
|
In-reply-to: Your message of Mon, 22 Mar 1999 21:25:45 -0500 (EST)
|
|
<199903230225.VAA01641@candle.pha.pa.us>
|
|
Date: Tue, 23 Mar 1999 12:09:52 -0500
|
|
Message-ID: <24471.922208992@sss.pgh.pa.us>
|
|
From: Tom Lane <tgl@sss.pgh.pa.us>
|
|
Sender: owner-pgsql-hackers@postgreSQL.org
|
|
Precedence: bulk
|
|
Status: RO
|
|
|
|
Bruce Momjian <maillist@candle.pha.pa.us> writes:
|
|
> What we really need is some way to determine how far the requested value
|
|
> is from the min/max values. With int, we just do (val-min)/(max-min).
|
|
> That works, but how do we do that for types that don't support division.
|
|
> Strings come to mind in this case.
|
|
|
|
What I'm envisioning is that we still apply the (val-min)/(max-min)
|
|
logic, but apply it to numeric values that are produced in a
|
|
type-dependent way.
|
|
|
|
For ints and floats the conversion is trivial, of course.
|
|
|
|
For strings, the first thing that comes to mind is to return 0 for a
|
|
null string and the value of the first byte for a non-null string.
|
|
This would give you one-part-in-256 selectivity which is plenty good
|
|
enough for what the selectivity code needs to do. (Actually, it's
|
|
only that good if the strings' first bytes are pretty well spread out.
|
|
If you have a table containing English words, for example, you might
|
|
only get about one part in 26 this way, since the first bytes will
|
|
probably only run from A to Z. Might be better to use the first two
|
|
characters of the string to compute the selectivity representation.)
|
|
|
|
In general, you can apply this logic as long as you can come up with
|
|
some numerical approximation to the data type's sorting order. It
|
|
doesn't have to be exact.
|
|
|
|
regards, tom lane
|
|
|
|
|
|
From owner-pgsql-hackers@hub.org Thu Jul 1 20:39:19 1999
|
|
Received: from hub.org (hub.org [209.167.229.1])
|
|
by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id UAA15403
|
|
for <maillist@candle.pha.pa.us>; Thu, 1 Jul 1999 20:39:18 -0400 (EDT)
|
|
Received: from hub.org (hub.org [209.167.229.1])
|
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|
|
Thu, 1 Jul 1999 20:20:27 -0400 (EDT)
|
|
(envelope-from owner-pgsql-hackers@hub.org)
|
|
Received: by hub.org (TLB v0.10a (1.23 tibbs 1997/01/09 00:29:32)); Thu, 01 Jul 1999 20:15:30 +0000 (EDT)
|
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|
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|
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|
|
(envelope-from owner-pgsql-hackers@postgreSQL.org)
|
|
X-Authentication-Warning: hub.org: majordom set sender to owner-pgsql-hackers@postgreSQL.org using -f
|
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|
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|
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for <pgsql-hackers@postgreSQL.org>; Thu, 1 Jul 1999 20:12:10 -0400 (EDT)
|
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(envelope-from Inoue@tpf.co.jp)
|
|
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|
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by sd.tpf.co.jp (2.5 Build 2640 (Berkeley 8.8.6)/8.8.4) with SMTP
|
|
id JAA00279 for <pgsql-hackers@postgreSQL.org>; Fri, 02 Jul 1999 09:11:58 +0900
|
|
From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
|
|
To: "pgsql-hackers" <pgsql-hackers@postgreSQL.org>
|
|
Subject: [HACKERS] Optimization FAQ ?
|
|
Date: Fri, 2 Jul 1999 09:14:10 +0900
|
|
Message-ID: <000401bec41f$ce81dcc0$2801007e@cadzone.tpf.co.jp>
|
|
MIME-Version: 1.0
|
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Content-Type: text/plain;
|
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charset="iso-2022-jp"
|
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|
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|
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|
|
Importance: Normal
|
|
Sender: owner-pgsql-hackers@postgreSQL.org
|
|
Precedence: bulk
|
|
Status: RO
|
|
|
|
Hello all,
|
|
|
|
I got the following result.
|
|
It's FAQ ?
|
|
|
|
drop table int2t;
|
|
create table int2t (id int2 primary key);
|
|
|
|
explain select * from int2t where id=1;
|
|
NOTICE: QUERY PLAN:
|
|
|
|
Seq Scan on int2t (cost=43.00 rows=2 width=2)
|
|
|
|
explain select * from int2t where id=1::int2;
|
|
NOTICE: QUERY PLAN:
|
|
|
|
Index Scan using int2t_pkey on int2t (cost=2.05 rows=2 width=2)
|
|
|
|
explain select * from int2t where id='1';
|
|
NOTICE: QUERY PLAN:
|
|
|
|
Index Scan using int2t_pkey on int2t (cost=2.05 rows=2 width=2)
|
|
|
|
Right behavior ?
|
|
|
|
Regards.
|
|
|
|
Hiroshi Inoue
|
|
Inoue@tpf.co.jp
|
|
|
|
|
|
From owner-pgsql-hackers@hub.org Thu Jan 20 18:45:32 2000
|
|
Received: from renoir.op.net (root@renoir.op.net [207.29.195.4])
|
|
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|
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for <pgman@candle.pha.pa.us>; Thu, 20 Jan 2000 19:45:30 -0500 (EST)
|
|
Received: from hub.org (hub.org [216.126.84.1]) by renoir.op.net (o1/$Revision: 1.19 $) with ESMTP id TAA01989 for <pgman@candle.pha.pa.us>; Thu, 20 Jan 2000 19:39:15 -0500 (EST)
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|
|
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|
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|
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|
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|
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|
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|
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|
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|
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for <pgsql-hackers@postgreSQL.org>; Thu, 20 Jan 2000 19:31:32 -0500 (EST)
|
|
To: pgsql-hackers@postgreSQL.org
|
|
Subject: [HACKERS] Some notes on optimizer cost estimates
|
|
Date: Thu, 20 Jan 2000 19:31:32 -0500
|
|
Message-ID: <25387.948414692@sss.pgh.pa.us>
|
|
From: Tom Lane <tgl@sss.pgh.pa.us>
|
|
Sender: owner-pgsql-hackers@postgreSQL.org
|
|
Status: OR
|
|
|
|
I have been spending some time measuring actual runtimes for various
|
|
sequential-scan and index-scan query plans, and have learned that the
|
|
current Postgres optimizer's cost estimation equations are not very
|
|
close to reality at all.
|
|
|
|
Presently we estimate the cost of a sequential scan as
|
|
|
|
Nblocks + CPU_PAGE_WEIGHT * Ntuples
|
|
|
|
--- that is, the unit of cost is the time to read one disk page,
|
|
and we have a "fudge factor" that relates CPU time per tuple to
|
|
disk time per page. (The default CPU_PAGE_WEIGHT is 0.033, which
|
|
is probably too high for modern hardware --- 0.01 seems like it
|
|
might be a better default, at least for simple queries.) OK,
|
|
it's a simplistic model, but not too unreasonable so far.
|
|
|
|
The cost of an index scan is measured in these same terms as
|
|
|
|
Nblocks + CPU_PAGE_WEIGHT * Ntuples +
|
|
CPU_INDEX_PAGE_WEIGHT * Nindextuples
|
|
|
|
Here Ntuples is the number of tuples selected by the index qual
|
|
condition (typically, it's less than the total table size used in
|
|
sequential-scan estimation). CPU_INDEX_PAGE_WEIGHT essentially
|
|
estimates the cost of scanning an index tuple; by default it's 0.017 or
|
|
half CPU_PAGE_WEIGHT. Nblocks is estimated as the index size plus an
|
|
appropriate fraction of the main table size.
|
|
|
|
There are two big problems with this:
|
|
|
|
1. Since main-table tuples are visited in index order, we'll be hopping
|
|
around from page to page in the table. The current cost estimation
|
|
method essentially assumes that the buffer cache plus OS disk cache will
|
|
be 100% efficient --- we will never have to read the same page of the
|
|
main table twice in a scan, due to having discarded it between
|
|
references. This of course is unreasonably optimistic. Worst case
|
|
is that we'd fetch a main-table page for each selected tuple, but in
|
|
most cases that'd be unreasonably pessimistic.
|
|
|
|
2. The cost of a disk page fetch is estimated at 1.0 unit for both
|
|
sequential and index scans. In reality, sequential access is *much*
|
|
cheaper than the quasi-random accesses performed by an index scan.
|
|
This is partly a matter of physical disk seeks, and partly a matter
|
|
of benefitting (or not) from any read-ahead logic the OS may employ.
|
|
|
|
As best I can measure on my hardware, the cost of a nonsequential
|
|
disk read should be estimated at 4 to 5 times the cost of a sequential
|
|
one --- I'm getting numbers like 2.2 msec per disk page for sequential
|
|
scans, and as much as 11 msec per page for index scans. I don't
|
|
know, however, if this ratio is similar enough on other platforms
|
|
to be useful for cost estimating. We could make it a parameter like
|
|
we do for CPU_PAGE_WEIGHT ... but you know and I know that no one
|
|
ever bothers to adjust those numbers in the field ...
|
|
|
|
The other effect that needs to be modeled, and currently is not, is the
|
|
"hit rate" of buffer cache. Presumably, this is 100% for tables smaller
|
|
than the cache and drops off as the table size increases --- but I have
|
|
no particular thoughts on the form of the dependency. Does anyone have
|
|
ideas here? The problem is complicated by the fact that we don't really
|
|
know how big the cache is; we know the number of buffers Postgres has,
|
|
but we have no idea how big a disk cache the kernel is keeping. As near
|
|
as I can tell, finding a hit in the kernel disk cache is not a lot more
|
|
expensive than having the page sitting in Postgres' own buffers ---
|
|
certainly it's much much cheaper than a disk read.
|
|
|
|
BTW, if you want to do some measurements of your own, try turning on
|
|
PGOPTIONS="-d 2 -te". This will dump a lot of interesting numbers
|
|
into the postmaster log, if your platform supports getrusage().
|
|
|
|
regards, tom lane
|
|
|
|
************
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From owner-pgsql-hackers@hub.org Thu Jan 20 20:26:33 2000
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To: pgsql-hackers@postgreSQL.org
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Reply-to: xun@cs.ucsb.edu
|
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Subject: Re. [HACKERS] Some notes on optimizer cost estimates
|
|
Date: Thu, 20 Jan 2000 18:19:40 -0800
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From: Xun Cheng <xun@cs.ucsb.edu>
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Sender: owner-pgsql-hackers@postgreSQL.org
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Status: OR
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I'm very glad you bring up this cost estimate issue.
|
|
Recent work in database research have argued a more
|
|
detailed disk access cost model should be used for
|
|
large queries especially joins.
|
|
Traditional cost estimate only considers the number of
|
|
disk pages accessed. However a more detailed model
|
|
would consider three parameters: avg. seek, avg. latency
|
|
and avg. page transfer. For old disk, typical values are
|
|
SEEK=9.5 milliseconds, LATENCY=8.3 ms, TRANSFER=2.6ms.
|
|
A sequential continuous reading of a table (assuming
|
|
1000 continuous pages) would cost
|
|
(SEEK+LATENCY+1000*TRANFER=2617.8ms); while quasi-randomly
|
|
reading 200 times with 2 continuous pages/time would
|
|
cost (SEEK+200*LATENCY+400*TRANSFER=2700ms).
|
|
Someone from IBM lab re-studied the traditional
|
|
ad hoc join algorithms (nested, sort-merge, hash) using the detailed cost model
|
|
and found some interesting results.
|
|
|
|
>I have been spending some time measuring actual runtimes for various
|
|
>sequential-scan and index-scan query plans, and have learned that the
|
|
>current Postgres optimizer's cost estimation equations are not very
|
|
>close to reality at all.
|
|
|
|
One interesting question I'd like to ask is if this non-closeness
|
|
really affects the optimal choice of postgresql's query optimizer.
|
|
And to what degree the effects might be? My point is that
|
|
if the optimizer estimated the cost for sequential-scan is 10 and
|
|
the cost for index-scan is 20 while the actual costs are 10 vs. 40,
|
|
it should be ok because the optimizer would still choose sequential-scan
|
|
as it should.
|
|
|
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>1. Since main-table tuples are visited in index order, we'll be hopping
|
|
>around from page to page in the table.
|
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|
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I'm not sure about the implementation in postgresql. One thing you might
|
|
be able to do is to first collect all must-read page addresses from
|
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the index scan and then order them before the actual ordered page fetching.
|
|
It would at least avoid the same page being read twice (not entirely
|
|
true depending on the context (like in join) and algo.)
|
|
|
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>The current cost estimation
|
|
>method essentially assumes that the buffer cache plus OS disk cache will
|
|
>be 100% efficient --- we will never have to read the same page of the
|
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>main table twice in a scan, due to having discarded it between
|
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>references. This of course is unreasonably optimistic. Worst case
|
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>is that we'd fetch a main-table page for each selected tuple, but in
|
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>most cases that'd be unreasonably pessimistic.
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|
|
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This is actually the motivation that I asked before if postgresql
|
|
has a raw disk facility. That way we have much control on this cache
|
|
issue. Of course only if we can provide some algo. better than OS
|
|
cache algo. (depending on the context, like large joins), a raw disk
|
|
facility will be worthwhile (besides the recoverability).
|
|
|
|
Actually I have another question for you guys which is somehow related
|
|
to this cost estimation issue. You know the difference between OLTP
|
|
and OLAP. My question is how you target postgresql on both kinds
|
|
of applications or just OLTP. From what I know OLTP and OLAP would
|
|
have a big difference in query characteristics and thus
|
|
optimization difference. If postgresql is only targeted on
|
|
OLTP, the above cost estimation issue might not be that
|
|
important. However for OLAP, large tables and large queries are
|
|
common and optimization would be difficult.
|
|
|
|
xun
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|
|
|
|
************
|
|
|
|
From owner-pgsql-hackers@hub.org Thu Jan 20 20:41:44 2000
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Thu, 20 Jan 2000 21:30:41 -0500 (EST)
|
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To: "Hiroshi Inoue" <Inoue@tpf.co.jp>
|
|
cc: pgsql-hackers@postgreSQL.org
|
|
Subject: Re: [HACKERS] Some notes on optimizer cost estimates
|
|
In-reply-to: <000b01bf63b1$093cbd40$2801007e@tpf.co.jp>
|
|
References: <000b01bf63b1$093cbd40$2801007e@tpf.co.jp>
|
|
Comments: In-reply-to "Hiroshi Inoue" <Inoue@tpf.co.jp>
|
|
message dated "Fri, 21 Jan 2000 10:44:20 +0900"
|
|
Date: Thu, 20 Jan 2000 21:30:41 -0500
|
|
Message-ID: <26758.948421841@sss.pgh.pa.us>
|
|
From: Tom Lane <tgl@sss.pgh.pa.us>
|
|
Sender: owner-pgsql-hackers@postgreSQL.org
|
|
Status: ORr
|
|
|
|
"Hiroshi Inoue" <Inoue@tpf.co.jp> writes:
|
|
> I've wondered why we cound't analyze database without vacuum.
|
|
> We couldn't run vacuum light-heartedly because it acquires an
|
|
> exclusive lock for the target table.
|
|
|
|
There is probably no real good reason, except backwards compatibility,
|
|
why the ANALYZE function (obtaining pg_statistic data) is part of
|
|
VACUUM at all --- it could just as easily be a separate command that
|
|
would only use read access on the database. Bruce is thinking about
|
|
restructuring VACUUM, so maybe now is a good time to think about
|
|
splitting out the ANALYZE code too.
|
|
|
|
> In addition,vacuum error occurs with analyze option in most
|
|
> cases AFAIK.
|
|
|
|
Still, with current sources? What's the error message? I fixed
|
|
a problem with pg_statistic tuples getting too big...
|
|
|
|
regards, tom lane
|
|
|
|
************
|
|
|
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From tgl@sss.pgh.pa.us Thu Jan 20 21:10:28 2000
|
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Received: from sss2.sss.pgh.pa.us (sss.pgh.pa.us [209.114.166.2])
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Thu, 20 Jan 2000 22:10:28 -0500 (EST)
|
|
To: Bruce Momjian <pgman@candle.pha.pa.us>
|
|
cc: Hiroshi Inoue <Inoue@tpf.co.jp>, pgsql-hackers@postgresql.org
|
|
Subject: Re: [HACKERS] Some notes on optimizer cost estimates
|
|
In-reply-to: <200001210248.VAA07186@candle.pha.pa.us>
|
|
References: <200001210248.VAA07186@candle.pha.pa.us>
|
|
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
|
|
message dated "Thu, 20 Jan 2000 21:48:57 -0500"
|
|
Date: Thu, 20 Jan 2000 22:10:28 -0500
|
|
Message-ID: <27077.948424228@sss.pgh.pa.us>
|
|
From: Tom Lane <tgl@sss.pgh.pa.us>
|
|
Status: OR
|
|
|
|
Bruce Momjian <pgman@candle.pha.pa.us> writes:
|
|
> It is nice that ANALYZE is done during vacuum. I can't imagine why you
|
|
> would want to do an analyze without adding a vacuum to it. I guess
|
|
> that's why I made them the same command.
|
|
|
|
Well, the main bad thing about ANALYZE being part of VACUUM is that
|
|
it adds to the length of time that VACUUM is holding an exclusive
|
|
lock on the table. I think it'd make more sense for it to be a
|
|
separate command.
|
|
|
|
I have also been thinking about how to make ANALYZE produce a more
|
|
reliable estimate of the most common value. The three-element list
|
|
that it keeps now is a good low-cost hack, but it really doesn't
|
|
produce a trustworthy answer unless the MCV is pretty darn C (since
|
|
it will never pick up on the MCV at all until there are at least
|
|
two occurrences in three adjacent tuples). The only idea I've come
|
|
up with is to use a larger list, which would be slower and take
|
|
more memory. I think that'd be OK in a separate command, but I
|
|
hesitate to do it inside VACUUM --- VACUUM has its own considerable
|
|
memory requirements, and there's still the issue of not holding down
|
|
an exclusive lock longer than you have to.
|
|
|
|
regards, tom lane
|
|
|
|
From Inoue@tpf.co.jp Thu Jan 20 21:08:32 2000
|
|
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id MAA04148; Fri, 21 Jan 2000 12:08:30 +0900
|
|
From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
|
|
To: "Bruce Momjian" <pgman@candle.pha.pa.us>, "Tom Lane" <tgl@sss.pgh.pa.us>
|
|
Cc: <pgsql-hackers@postgreSQL.org>
|
|
Subject: RE: [HACKERS] Some notes on optimizer cost estimates
|
|
Date: Fri, 21 Jan 2000 12:14:10 +0900
|
|
Message-ID: <001301bf63bd$95cbe680$2801007e@tpf.co.jp>
|
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MIME-Version: 1.0
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|
|
Importance: Normal
|
|
Status: OR
|
|
|
|
> -----Original Message-----
|
|
> From: Bruce Momjian [mailto:pgman@candle.pha.pa.us]
|
|
>
|
|
> > "Hiroshi Inoue" <Inoue@tpf.co.jp> writes:
|
|
> > > I've wondered why we cound't analyze database without vacuum.
|
|
> > > We couldn't run vacuum light-heartedly because it acquires an
|
|
> > > exclusive lock for the target table.
|
|
> >
|
|
> > There is probably no real good reason, except backwards compatibility,
|
|
> > why the ANALYZE function (obtaining pg_statistic data) is part of
|
|
> > VACUUM at all --- it could just as easily be a separate command that
|
|
> > would only use read access on the database. Bruce is thinking about
|
|
> > restructuring VACUUM, so maybe now is a good time to think about
|
|
> > splitting out the ANALYZE code too.
|
|
>
|
|
> I put it in vacuum because at the time I didn't know how to do such
|
|
> things and vacuum already scanned the table. I just linked on the the
|
|
> scan. Seemed like a good idea at the time.
|
|
>
|
|
> It is nice that ANALYZE is done during vacuum. I can't imagine why you
|
|
> would want to do an analyze without adding a vacuum to it. I guess
|
|
> that's why I made them the same command.
|
|
>
|
|
> If I made them separate commands, both would have to scan the table,
|
|
> though the analyze could do it without the exclusive lock, which would
|
|
> be good.
|
|
>
|
|
|
|
The functionality of VACUUM and ANALYZE is quite different.
|
|
I don't prefer to charge VACUUM more than now about analyzing
|
|
database. Probably looong lock,more aborts ....
|
|
Various kind of analysis would be possible by splitting out ANALYZE.
|
|
|
|
Regards.
|
|
|
|
Hiroshi Inoue
|
|
Inoue@tpf.co.jp
|
|
|
|
From owner-pgsql-hackers@hub.org Fri Jan 21 11:01:59 2000
|
|
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Date: Fri, 21 Jan 2000 08:10:44 -0800
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To: xun@cs.ucsb.edu, pgsql-hackers@postgreSQL.org
|
|
From: Don Baccus <dhogaza@pacifier.com>
|
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Subject: Re: Re. [HACKERS] Some notes on optimizer cost estimates
|
|
In-Reply-To: <200001210219.SAA22377@xp10-06.dialup.commserv.ucsb.edu>
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Mime-Version: 1.0
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Sender: owner-pgsql-hackers@postgreSQL.org
|
|
Status: OR
|
|
|
|
At 06:19 PM 1/20/00 -0800, Xun Cheng wrote:
|
|
>I'm very glad you bring up this cost estimate issue.
|
|
>Recent work in database research have argued a more
|
|
>detailed disk access cost model should be used for
|
|
>large queries especially joins.
|
|
>Traditional cost estimate only considers the number of
|
|
>disk pages accessed. However a more detailed model
|
|
>would consider three parameters: avg. seek, avg. latency
|
|
>and avg. page transfer. For old disk, typical values are
|
|
>SEEK=9.5 milliseconds, LATENCY=8.3 ms, TRANSFER=2.6ms.
|
|
>A sequential continuous reading of a table (assuming
|
|
>1000 continuous pages) would cost
|
|
>(SEEK+LATENCY+1000*TRANFER=2617.8ms); while quasi-randomly
|
|
>reading 200 times with 2 continuous pages/time would
|
|
>cost (SEEK+200*LATENCY+400*TRANSFER=2700ms).
|
|
>Someone from IBM lab re-studied the traditional
|
|
>ad hoc join algorithms (nested, sort-merge, hash) using the detailed cost
|
|
model
|
|
>and found some interesting results.
|
|
|
|
One complication when doing an index scan is that you are
|
|
accessing two separate files (table and index), which can frequently
|
|
be expected to cause an considerable increase in average seek time.
|
|
|
|
Oracle and other commercial databases recommend spreading indices and
|
|
tables over several spindles if at all possible in order to minimize
|
|
this effect.
|
|
|
|
I suspect it also helps their optimizer make decisions that are
|
|
more consistently good for customers with the largest and most
|
|
complex databases and queries, by making cost estimates more predictably
|
|
reasonable.
|
|
|
|
Still...this doesn't help with the question about the effect of the
|
|
filesystem system cache. I wandered around the web for a little bit
|
|
last night, and found one summary of a paper by Osterhout on the
|
|
effect of the Solaris cache on a fileserver serving diskless workstations.
|
|
There was reference to the hierarchy involved (i.e. the local workstation
|
|
cache is faster than the fileserver's cache which has to be read via
|
|
the network which in turn is faster than reading from the fileserver's
|
|
disk). It appears the rule-of-thumb for the cache-hit ratio on reads,
|
|
presumably based on measuring some internal Sun systems, used in their
|
|
calculations was 80%.
|
|
|
|
Just a datapoint to think about.
|
|
|
|
There's also considerable operating system theory on paging systems
|
|
that might be useful for thinking about trying to estimate the
|
|
Postgres cache/hit ratio. Then again, maybe Postgres could just
|
|
keep count of how many pages of a given table are in the cache at
|
|
any given time? Or simply keep track of the current ratio of hits
|
|
and misses?
|
|
|
|
>>I have been spending some time measuring actual runtimes for various
|
|
>>sequential-scan and index-scan query plans, and have learned that the
|
|
>>current Postgres optimizer's cost estimation equations are not very
|
|
>>close to reality at all.
|
|
|
|
>One interesting question I'd like to ask is if this non-closeness
|
|
>really affects the optimal choice of postgresql's query optimizer.
|
|
>And to what degree the effects might be? My point is that
|
|
>if the optimizer estimated the cost for sequential-scan is 10 and
|
|
>the cost for index-scan is 20 while the actual costs are 10 vs. 40,
|
|
>it should be ok because the optimizer would still choose sequential-scan
|
|
>as it should.
|
|
|
|
This is crucial, of course - if there are only two types of scans
|
|
available, what ever heuristic is used only has to be accurate enough
|
|
to pick the right one. Once the choice is made, it doesn't really
|
|
matter (from the optimizer's POV) just how long it will actually take,
|
|
the time will be spent and presumably it will be shorter than the
|
|
alternative.
|
|
|
|
How frequently will the optimizer choose wrongly if:
|
|
|
|
1. All of the tables and indices were in PG buffer cache or filesystem
|
|
cache? (i.e. fixed access times for both types of scans)
|
|
|
|
or
|
|
|
|
2. The table's so big that only a small fraction can reside in RAM
|
|
during the scan and join, which means that the non-sequential
|
|
disk access pattern of the indexed scan is much more expensive.
|
|
|
|
Also, if you pick sequential scans more frequently based on a presumption
|
|
that index scans are expensive due to increased average seek time, how
|
|
often will this penalize the heavy-duty user that invests in extra
|
|
drives and lots of RAM?
|
|
|
|
...
|
|
|
|
>>The current cost estimation
|
|
>>method essentially assumes that the buffer cache plus OS disk cache will
|
|
>>be 100% efficient --- we will never have to read the same page of the
|
|
>>main table twice in a scan, due to having discarded it between
|
|
>>references. This of course is unreasonably optimistic. Worst case
|
|
>>is that we'd fetch a main-table page for each selected tuple, but in
|
|
>>most cases that'd be unreasonably pessimistic.
|
|
>
|
|
>This is actually the motivation that I asked before if postgresql
|
|
>has a raw disk facility. That way we have much control on this cache
|
|
>issue. Of course only if we can provide some algo. better than OS
|
|
>cache algo. (depending on the context, like large joins), a raw disk
|
|
>facility will be worthwhile (besides the recoverability).
|
|
|
|
Postgres does have control over its buffer cache. The one thing that
|
|
raw disk I/O would give you is control over where blocks are placed,
|
|
meaning you could more accurately model the cost of retrieving them.
|
|
So presumably the cache could be tuned to the allocation algorithm
|
|
used to place various structures on the disk.
|
|
|
|
I still wonder just how much gain you get by this approach. Compared,
|
|
to, say simply spending $2,000 on a gigabyte of RAM. Heck, PCs even
|
|
support a couple gigs of RAM now.
|
|
|
|
>Actually I have another question for you guys which is somehow related
|
|
>to this cost estimation issue. You know the difference between OLTP
|
|
>and OLAP. My question is how you target postgresql on both kinds
|
|
>of applications or just OLTP. From what I know OLTP and OLAP would
|
|
>have a big difference in query characteristics and thus
|
|
>optimization difference. If postgresql is only targeted on
|
|
>OLTP, the above cost estimation issue might not be that
|
|
>important. However for OLAP, large tables and large queries are
|
|
>common and optimization would be difficult.
|
|
|
|
|
|
|
|
- Don Baccus, Portland OR <dhogaza@pacifier.com>
|
|
Nature photos, on-line guides, Pacific Northwest
|
|
Rare Bird Alert Service and other goodies at
|
|
http://donb.photo.net.
|
|
|
|
************
|
|
|
|
From pgsql-hackers-owner+M6019@hub.org Mon Aug 21 11:47:56 2000
|
|
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|
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Date: Mon, 21 Aug 2000 16:48:08 +0100 (WEST)
|
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From: =?iso-8859-1?Q?Tiago_Ant=E3o?= <tra@fct.unl.pt>
|
|
X-Sender: tiago@eros.si.fct.unl.pt
|
|
To: Tom Lane <tgl@sss.pgh.pa.us>
|
|
cc: pgsql-hackers@postgresql.org
|
|
Subject: Re: [HACKERS] Optimisation deficiency: currval('seq')-->seq scan,
|
|
constant-->index scan
|
|
In-Reply-To: <1731.966868649@sss.pgh.pa.us>
|
|
Message-ID: <Pine.LNX.4.21.0008211626250.25226-100000@eros.si.fct.unl.pt>
|
|
MIME-Version: 1.0
|
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Content-Type: TEXT/PLAIN; charset=US-ASCII
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|
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Status: ORr
|
|
|
|
On Mon, 21 Aug 2000, Tom Lane wrote:
|
|
|
|
> > One thing it might be interesting (please tell me if you think
|
|
> > otherwise) would be to improve pg with better statistical information, by
|
|
> > using, for example, histograms.
|
|
>
|
|
> Yes, that's been on the todo list for a while.
|
|
|
|
If it's ok and nobody is working on that, I'll look on that subject.
|
|
I'll start by looking at the analize portion of vacuum. I'm thinking in
|
|
using arrays for the histogram (I've never used the array data type of
|
|
postgres).
|
|
Should I use 7.0.2 or the cvs version?
|
|
|
|
|
|
> Interesting article. We do most of what she talks about, but we don't
|
|
> have anything like the ClusterRatio statistic. We need it --- that was
|
|
> just being discussed a few days ago in another thread. Do you have any
|
|
> reference on exactly how DB2 defines that stat?
|
|
|
|
|
|
I don't remember seeing that information spefically. From what I've
|
|
read I can speculate:
|
|
|
|
1. They have clusterratios for both indexes and the relation itself.
|
|
2. They might use an index even if there is no "order by" if the table
|
|
has a low clusterratio: just to get the RIDs, then sort the RIDs and
|
|
fetch.
|
|
3. One possible way to calculate this ratio:
|
|
a) for tables
|
|
SeqScan
|
|
if tuple points to a next tuple on the same page then its
|
|
"good"
|
|
ratio = # good tuples / # all tuples
|
|
b) for indexes (high speculation ratio here)
|
|
foreach pointed RID in index
|
|
if RID is in same page of next RID in index than mark as
|
|
"good"
|
|
|
|
I suspect that if a tuple size is big (relative to page size) than the
|
|
cluster ratio is always low.
|
|
|
|
A tuple might also be "good" if it pointed to the next page.
|
|
|
|
Tiago
|
|
|
|
|
|
From pgsql-hackers-owner+M6152@hub.org Wed Aug 23 13:00:33 2000
|
|
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by fct1.si.fct.unl.pt with SMTP; 23 Aug 2000 15:10:04 -0000
|
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Date: Wed, 23 Aug 2000 16:03:42 +0100 (WEST)
|
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From: =?iso-8859-1?Q?Tiago_Ant=E3o?= <tra@fct.unl.pt>
|
|
X-Sender: tiago@eros.si.fct.unl.pt
|
|
To: Tom Lane <tgl@sss.pgh.pa.us>
|
|
cc: Jules Bean <jules@jellybean.co.uk>, pgsql-hackers@postgresql.org
|
|
Subject: Re: [HACKERS] Optimisation deficiency: currval('seq')-->seq scan,
|
|
constant-->index scan
|
|
In-Reply-To: <27971.967041030@sss.pgh.pa.us>
|
|
Message-ID: <Pine.LNX.4.21.0008231543340.4273-100000@eros.si.fct.unl.pt>
|
|
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|
Status: ORr
|
|
|
|
Hi!
|
|
|
|
On Wed, 23 Aug 2000, Tom Lane wrote:
|
|
|
|
> Yes, we know about that one. We have stats about the most common value
|
|
> in a column, but no information about how the less-common values are
|
|
> distributed. We definitely need stats about several top values not just
|
|
> one, because this phenomenon of a badly skewed distribution is pretty
|
|
> common.
|
|
|
|
|
|
An end-biased histogram has stats on top values and also on the least
|
|
frequent values. So if a there is a selection on a value that is well
|
|
bellow average, the selectivity estimation will be more acurate. On some
|
|
research papers I've read, it's refered that this is a better approach
|
|
than equi-width histograms (which are said to be the "industry" standard).
|
|
|
|
I not sure whether to use a table or a array attribute on pg_stat for
|
|
the histogram, the problem is what could be expected from the size of the
|
|
attribute (being a text). I'm very affraid of the cost of going through
|
|
several tuples on a table (pg_histogram?) during the optimization phase.
|
|
|
|
One other idea would be to only have better statistics for special
|
|
attributes requested by the user... something like "analyze special
|
|
table(column)".
|
|
|
|
Best Regards,
|
|
Tiago
|
|
|
|
|
|
|
|
From pgsql-hackers-owner+M6160@hub.org Thu Aug 24 00:21:39 2000
|
|
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Wed, 23 Aug 2000 23:56:35 -0400 (EDT)
|
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To: =?iso-8859-1?Q?Tiago_Ant=E3o?= <tra@fct.unl.pt>
|
|
cc: Jules Bean <jules@jellybean.co.uk>, pgsql-hackers@postgresql.org
|
|
Subject: Re: [HACKERS] Optimisation deficiency: currval('seq')-->seq scan, constant-->index scan
|
|
In-reply-to: <Pine.LNX.4.21.0008231543340.4273-100000@eros.si.fct.unl.pt>
|
|
References: <Pine.LNX.4.21.0008231543340.4273-100000@eros.si.fct.unl.pt>
|
|
Comments: In-reply-to =?iso-8859-1?Q?Tiago_Ant=E3o?= <tra@fct.unl.pt>
|
|
message dated "Wed, 23 Aug 2000 16:03:42 +0100"
|
|
Date: Wed, 23 Aug 2000 23:56:35 -0400
|
|
Message-ID: <20970.967089395@sss.pgh.pa.us>
|
|
From: Tom Lane <tgl@sss.pgh.pa.us>
|
|
X-Mailing-List: pgsql-hackers@postgresql.org
|
|
Precedence: bulk
|
|
Sender: pgsql-hackers-owner@hub.org
|
|
Status: OR
|
|
|
|
=?iso-8859-1?Q?Tiago_Ant=E3o?= <tra@fct.unl.pt> writes:
|
|
> One other idea would be to only have better statistics for special
|
|
> attributes requested by the user... something like "analyze special
|
|
> table(column)".
|
|
|
|
This might actually fall out "for free" from the cheapest way of
|
|
implementing the stats. We've talked before about scanning btree
|
|
indexes directly to obtain data values in sorted order, which makes
|
|
it very easy to find the most common values. If you do that, you
|
|
get good stats for exactly those columns that the user has created
|
|
indexes on. A tad indirect but I bet it'd be effective...
|
|
|
|
regards, tom lane
|
|
|
|
From pgsql-hackers-owner+M6165@hub.org Thu Aug 24 05:33:02 2000
|
|
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|
|
Date: Thu, 24 Aug 2000 10:11:14 +0100
|
|
From: Jules Bean <jules@jellybean.co.uk>
|
|
To: Tom Lane <tgl@sss.pgh.pa.us>
|
|
Cc: Tiago Ant?o <tra@fct.unl.pt>, pgsql-hackers@postgresql.org
|
|
Subject: Re: [HACKERS] Optimisation deficiency: currval('seq')-->seq scan, constant-->index scan
|
|
Message-ID: <20000824101113.N17510@grommit.office.vi.net>
|
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References: <1731.966868649@sss.pgh.pa.us> <Pine.LNX.4.21.0008211626250.25226-100000@eros.si.fct.unl.pt> <20000823133418.F17510@grommit.office.vi.net> <27971.967041030@sss.pgh.pa.us>
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In-Reply-To: <27971.967041030@sss.pgh.pa.us>; from tgl@sss.pgh.pa.us on Wed, Aug 23, 2000 at 10:30:30AM -0400
|
|
X-Mailing-List: pgsql-hackers@postgresql.org
|
|
Precedence: bulk
|
|
Sender: pgsql-hackers-owner@hub.org
|
|
Status: OR
|
|
|
|
On Wed, Aug 23, 2000 at 10:30:30AM -0400, Tom Lane wrote:
|
|
> Jules Bean <jules@jellybean.co.uk> writes:
|
|
> > I have in a table a 'category' column which takes a small number of
|
|
> > (basically fixed) values. Here by 'small', I mean ~1000, while the
|
|
> > table itself has ~10 000 000 rows. Some categories have many, many
|
|
> > more rows than others. In particular, there's one category which hits
|
|
> > over half the rows. Because of this (AIUI) postgresql assumes
|
|
> > that the query
|
|
> > select ... from thistable where category='something'
|
|
> > is best served by a seqscan, even though there is an index on
|
|
> > category.
|
|
>
|
|
> Yes, we know about that one. We have stats about the most common value
|
|
> in a column, but no information about how the less-common values are
|
|
> distributed. We definitely need stats about several top values not just
|
|
> one, because this phenomenon of a badly skewed distribution is pretty
|
|
> common.
|
|
|
|
ISTM that that might be enough, in fact.
|
|
|
|
If you have stats telling you that the most popular value is 'xyz',
|
|
and that it constitutes 50% of the rows (i.e. 5 000 000) then you can
|
|
conclude that, on average, other entries constitute a mere 5 000
|
|
000/999 ~~ 5000 entries, and it would be definitely be enough.
|
|
(That's assuming you store the number of distinct values somewhere).
|
|
|
|
|
|
> BTW, if your highly-popular value is actually a dummy value ('UNKNOWN'
|
|
> or something like that), a fairly effective workaround is to replace the
|
|
> dummy entries with NULL. The system does account for NULLs separately
|
|
> from real values, so you'd then get stats based on the most common
|
|
> non-dummy value.
|
|
|
|
I can't really do that. Even if I could, the distribution is very
|
|
skewed -- so the next most common makes up a very high proportion of
|
|
what's left. I forget the figures exactly.
|
|
|
|
Jules
|
|
|
|
From pgsql-hackers-owner+M6154@hub.org Wed Aug 23 14:36:41 2000
|
|
Received: from hub.org (root@hub.org [216.126.84.1])
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Date: Wed, 23 Aug 2000 18:22:40 +0100 (WEST)
|
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From: =?iso-8859-1?Q?Tiago_Ant=E3o?= <tra@fct.unl.pt>
|
|
X-Sender: tiago@eros.si.fct.unl.pt
|
|
To: Tom Lane <tgl@sss.pgh.pa.us>
|
|
cc: =?iso-8859-1?Q?Tiago_Ant=E3o?= <tra@fct.unl.pt>,
|
|
PostgreSQL Hackers list <pgsql-hackers@hub.org>
|
|
Subject: Re: [HACKERS] analyze.c
|
|
In-Reply-To: <28154.967041988@sss.pgh.pa.us>
|
|
Message-ID: <Pine.LNX.4.21.0008231742420.5111-100000@eros.si.fct.unl.pt>
|
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|
|
|
|
|
|
|
|
On Wed, 23 Aug 2000, Tom Lane wrote:
|
|
|
|
> > What's the big reason not to do that? I know that
|
|
> > there is some code in analyze.c (like comparing) that uses other parts of
|
|
> > pg, but that seems to be easily fixed.
|
|
>
|
|
> Are you proposing not to do any comparisons? It will be interesting to
|
|
> see how you can compute a histogram without any idea of equality or
|
|
> ordering. But if you want that, then you still need the function-call
|
|
> manager as well as the type-specific comparison routines for every
|
|
> datatype that you might be asked to operate on (don't forget
|
|
> user-defined types here).
|
|
|
|
I forgot user defined data types :-(, but regarding histograms I think
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the code can be made external (at least for testing purposes):
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1. I was not suggesting not to do any comparisons, but I think the only
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comparison I need is equality, I don't need order as I don't need to
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calculate mins or maxs (I just need mins and maxes on frequencies, NOT on
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dat itself) to make a histogram.
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2. The mapping to text guarantees that I have (PQgetvalue returns
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always char* and pg_statistics keeps a "text" anyway) a way of knowing
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about equality regardless of type.
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But at least anything relating to order has to be in.
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> > I'm leaning toward the implementation of end-biased histograms. There is
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> > an introductory reference in the IEEE Data Engineering Bulletin, september
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> > 1995 (available on microsoft research site).
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>
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> Sounds interesting. Can you give us an exact URL?
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http://www.research.microsoft.com/research/db/debull/default.htm
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BTW, you can get access to SIGMOD CDs with lots of goodies for a very low
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price (at least in 1999 it was a bargain), check out ACM membership for
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sigmod.
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I've been reading something about implementation of histograms, and,
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AFAIK, in practice histograms is just a cool name for no more than:
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1. top ten with frequency for each
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2. the same for top ten worse
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3. average for the rest
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I'm writing code get this info (outside pg for now - for testing
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purposes).
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Best Regards,
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Tiago
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PS - again: I'm starting, so, some of my comments can be completly dumb.
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From pgsql-hackers-owner+M7514@hub.org Sun Oct 15 20:38:12 2000
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Received: from hub.org (hub.org [216.126.84.1])
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for <pgman@candle.pha.pa.us>; Sun, 15 Oct 2000 19:38:12 -0400 (EDT)
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Sun, 15 Oct 2000 19:33:36 -0400 (EDT)
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To: Bruce Momjian <pgman@candle.pha.pa.us>
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cc: Jules Bean <jules@jellybean.co.uk>,
|
|
Alfred Perlstein <bright@wintelcom.net>, pgsql-hackers@postgresql.org
|
|
Subject: Re: [HACKERS] Performance on inserts
|
|
In-reply-to: <200010152320.TAA17944@candle.pha.pa.us>
|
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References: <200010152320.TAA17944@candle.pha.pa.us>
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Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
|
|
message dated "Sun, 15 Oct 2000 19:20:35 -0400"
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Date: Sun, 15 Oct 2000 19:33:36 -0400
|
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Message-ID: <6043.971652816@sss.pgh.pa.us>
|
|
From: Tom Lane <tgl@sss.pgh.pa.us>
|
|
X-Mailing-List: pgsql-hackers@postgresql.org
|
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Precedence: bulk
|
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Sender: pgsql-hackers-owner@hub.org
|
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Status: ORr
|
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|
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Bruce Momjian <pgman@candle.pha.pa.us> writes:
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> However, assume tab2.col2 equals 3. I assume this would cause an index
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> scan because the executor doesn't know about the most common value,
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> right? Is it worth trying to improve that?
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Oh, I see: you are assuming that a nestloop join is being done, and
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wondering if it's worthwhile to switch dynamically between seqscan
|
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and indexscan for each scan of the inner relation, depending on exactly
|
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what value is being supplied from the outer relation for that scan.
|
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Hmm.
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Not sure if it's worth the trouble or not. Nestloop is usually a
|
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last-resort join strategy anyway, and is unlikely to be picked when the
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tables are large enough to make performance be a big issue.
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|
|
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regards, tom lane
|
|
|
|
From tgl@sss.pgh.pa.us Mon Oct 16 01:48:27 2000
|
|
Received: from sss.pgh.pa.us (sss.pgh.pa.us [209.114.132.154])
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for <pgman@candle.pha.pa.us>; Mon, 16 Oct 2000 00:48:26 -0400 (EDT)
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|
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by sss.pgh.pa.us (8.11.1/8.11.1) with ESMTP id e9G4mu521809;
|
|
Mon, 16 Oct 2000 00:48:56 -0400 (EDT)
|
|
To: Bruce Momjian <pgman@candle.pha.pa.us>
|
|
cc: Jules Bean <jules@jellybean.co.uk>,
|
|
Alfred Perlstein <bright@wintelcom.net>, pgsql-hackers@postgresql.org
|
|
Subject: Re: [HACKERS] Performance on inserts
|
|
In-reply-to: <200010160441.AAA01374@candle.pha.pa.us>
|
|
References: <200010160441.AAA01374@candle.pha.pa.us>
|
|
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
|
|
message dated "Mon, 16 Oct 2000 00:41:49 -0400"
|
|
Date: Mon, 16 Oct 2000 00:48:56 -0400
|
|
Message-ID: <21806.971671736@sss.pgh.pa.us>
|
|
From: Tom Lane <tgl@sss.pgh.pa.us>
|
|
Status: OR
|
|
|
|
Bruce Momjian <pgman@candle.pha.pa.us> writes:
|
|
>> So an inner indexscan for tab1 is definitely a possible plan.
|
|
|
|
> Yes, that was my point, that a nested loop could easily be involved if
|
|
> the joined table has a restriction. Is there a TODO item here?
|
|
|
|
More like a "to investigate" --- I'm not sold on the idea that a
|
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dynamic switch in plan types would be a win. Maybe it would be,
|
|
but...
|
|
|
|
One thing to think about is that it'd be critically dependent on having
|
|
accurate statistics. Currently, the planner only places bets on the
|
|
average behavior over a whole join. If you make a separate bet on each
|
|
scan, then you open up the risk of betting wrong every time, should
|
|
your stats be out-of-date or otherwise misleading.
|
|
|
|
regards, tom lane
|
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|