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postgresql/doc/TODO.detail/count
Bruce Momjian ea079e27b1 Add count(*) TODO detail.
2003-10-10 01:53:06 +00:00

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From pgsql-performance-owner+M3897@postgresql.org Sat Oct 4 19:50:57 2003
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From: Christopher Browne <cbbrowne@acm.org>
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Subject: Re: [PERFORM] count(*) slow on large tables
Date: Sat, 04 Oct 2003 19:33:46 -0400
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Quoth tgl@sss.pgh.pa.us (Tom Lane):
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
>> We do have a TODO item:
>> * Consider using MVCC to cache count(*) queries with no WHERE clause
>
>> The idea is to cache a recent count of the table, then have
>> insert/delete add +/- records to the count. A COUNT(*) would get the
>> main cached record plus any visible +/- records. This would allow the
>> count to return the proper value depending on the visibility of the
>> requesting transaction, and it would require _no_ heap or index scan.
>
> ... and it would give the wrong answers. Unless the cache is somehow
> snapshot-aware, so that it can know which other transactions should be
> included in your count.
[That's an excellent summary that Bruce did of what came out of the
previous discussion...]
If this "cache" was a table, itself, the visibility of its records
should be identical to that of the visibility of the "real" records.
+/- records would become visible when the transaction COMMITed, at the
very same time the source records became visible.
I thought, at one point, that it would be a slick idea for "record
compression" to take place automatically; when you do a COUNT(*), the
process would include compressing multiple records down to one.
Unfortunately, that turns out to be Tremendously Evil if the same
COUNT(*) were being concurrently processed in multiple transactions.
Both would repeat much the same work, and this would ultimately lead
to one of the transactions aborting. [I recently saw this effect
occur, um, a few times...]
For this not to have Evil Effects on unsuspecting transactions, we
would instead require some process analagous to VACUUM, where a single
transaction would be used to compress the "counts table" down to one
record per table. Being independent of "user transactions," it could
safely compress the data without injuring unsuspecting transactions.
But in most cases, the cost of this would be pretty prohibitive.
Every transaction that adds a record to a table leads to a record
being added to table "pg_exact_row_counts". If transactions typically
involve adding ONE row to any given table, this effectively doubles
the update traffic. Ouch. That means that in a _real_
implementation, it would make sense to pick and choose the tables that
would be so managed.
In my earlier arguing of "You don't really want that!", while I may
have been guilty of engaging in a _little_ hyperbole, I was certainly
_not_ being facetious overall. At work, we tell the developers "avoid
doing COUNT(*) inside ordinary transactions!", and that is certainly
NOT facetious comment. I recall a case a while back where system
performance was getting brutalized by a lurking COUNT(*). (Combined
with some other pathological behaviour, of course!) And note that
this wasn't a query that the TODO item could address; it was of the
form "SELECT COUNT(*) FROM SOME_TABLE WHERE OWNER = VALUE;"
As you have commented elsewhere in the thread, much of the time, the
point of asking for COUNT(*) is often to get some idea of table size,
where the precise number isn't terribly important in comparison with
getting general magnitude. Improving the ability to get approximate
values would be of some value.
I would further argue that "SELECT COUNT(*) FROM TABLE" isn't
particularly useful even when precision _is_ important. If I'm
working on reports that would be used to reconcile things, the queries
I use are a whole lot more involved than that simple form. It is far
more likely that I'm using a GROUP BY.
It is legitimate to get wishful and imagine that it would be nice if
we could get the value of that query "instantaneously." It is also
legitimate to think that the effort required to implement that might
be better used on improving other things.
--
(reverse (concatenate 'string "ac.notelrac.teneerf" "@" "454aa"))
http://www3.sympatico.ca/cbbrowne/
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From: Josh Berkus <josh@agliodbs.com>
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To: Bruce Momjian <pgman@candle.pha.pa.us>, Tom Lane <tgl@sss.pgh.pa.us>
Subject: Re: [PERFORM] count(*) slow on large tables
Date: Sun, 5 Oct 2003 11:57:21 -0700
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Bruce,
> OK, I beefed up the TODO:
>
> * Use a fixed row count and a +/- count with MVCC visibility rules
> to allow fast COUNT(*) queries with no WHERE clause(?)
>
> I can always give the details if someone asks. It doesn't seem complex
> enough for a separate TODO.detail item.
Hmmm ... this doesn't seem effort-worthy to me. How often does anyone do
COUNT with no where clause, except GUIs that give you a record count? (of
course, as always, if someone wants to code it, feel free ...)
And for those GUIs, wouldn't it be 97% as good to run an ANALYZE and give the
approximate record counts for large tables?
As for counts with a WHERE clause, this is obviously up to the user. Joe
Conway and I tested using a C trigger to track some COUNT ... GROUP BY values
for large tables based on additive numbers. It worked fairly well for
accuracy, but the performance penalty on data writes was significant ... 8%
to 25% penalty for UPDATES, depending on the frequency and batch size (>
frequency > batch size --> > penalty)
It's possible that this could be improved through some mechanism more tightly
integrated with the source code. However,the coding effort would be
significant ( 12-20 hours ) and it's possible that there would be no
improvement, which is why we didn't do it.
We also discussed an asynchronous aggregates collector that would work
something like the statistics collector, and keep pre-programmmed aggregate
data, updating during "low-activity" periods. This would significantly
reduce the performance penalty, but at the cost of accuracy ... that is, a
1%-5% variance on high-activity tables would be unavoidable, and all cached
aggregates would have to be recalculated on database restart, significantly
slowing down startup. Again, we felt that the effort-result payoff was not
worthwhile.
Overall, I think the stuff we already have planned ... the hash aggregates in
7.4 and Tom's suggestion of adding an indexable flag to pg_aggs ... are far
more likely to yeild useful fruit than any caching plan.
--
Josh Berkus
Aglio Database Solutions
San Francisco
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From: Shridhar Daithankar <shridhar_daithankar@persistent.co.in>
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Subject: Re: [PERFORM] count(*) slow on large tables
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Bruce Momjian wrote:
> OK, I beefed up the TODO:
>
> * Use a fixed row count and a +/- count with MVCC visibility rules
> to allow fast COUNT(*) queries with no WHERE clause(?)
>
> I can always give the details if someone asks. It doesn't seem complex
> enough for a separate TODO.detail item.
May I propose alternate approach for this optimisation?
- Postgresql allows to maintain user defined variables in shared memory.
- These variables obey transactions but do not get written to disk at all.
- There should be a facility to detect whether such a variable is initialized or
not.
How it will help? This is in addition to trigger proposal that came up earlier.
With triggers it's not possible to make values visible across backends unless
trigger updates a table, which eventually leads to vacuum/dead tuples problem.
1. User creates a trigger to check updates/inserts for certain conditions.
2. It updates the count as and when required.
3. If the trigger detects the count is not initialized, it would issue the same
query first time. There is no avoiding this issue.
Besides providing facility of resident variables could be used imaginatively as
well.
Does this make sense? IMO this is more generalised approach over all.
Just a thought.
Shridhar
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From: Harald Fuchs <nospam@sap.com>
Subject: Re: [PERFORM] count(*) slow on large tables
Date: 06 Oct 2003 17:08:36 +0200
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In article <3F7D172E.3060107@persistent.co.in>,
Shridhar Daithankar <shridhar_daithankar@persistent.co.in> writes:
> Dror Matalon wrote:
>> I smell a religious war in the aii:-). Can you go several days in a
>> row without doing select count(*) on any
>> of your tables? I suspect that this is somewhat a domain specific
>> issue. In some areas
>> you don't need to know the total number of rows in your tables, in
>> others you do.
> If I were you, I would have an autovacuum daemon running and rather
> than doing select count(*), I would look at stats generated by
> vacuums. They give approximate number of tuples and it should be good
> enough it is accurate within a percent.
The stats might indeed be a good estimate presumed there were not many
changes since the last VACUUM. But how about a variant of COUNT(*)
using an index? It would not be quite exact since it might contain
tuples not visible in the current transaction, but it might be a much
better estimate than the stats.
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From: Sean Chittenden <sean@chittenden.org>
To: Shridhar Daithankar <shridhar_daithankar@persistent.co.in>
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Subject: Re: [PERFORM] count(*) slow on large tables
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> How it will help? This is in addition to trigger proposal that came
> up earlier. With triggers it's not possible to make values visible
> across backends unless trigger updates a table, which eventually
> leads to vacuum/dead tuples problem.
>
> 1. User creates a trigger to check updates/inserts for certain conditions.
> 2. It updates the count as and when required.
> 3. If the trigger detects the count is not initialized, it would issue the
> same query first time. There is no avoiding this issue.
>
> Besides providing facility of resident variables could be used
> imaginatively as well.
>
> Does this make sense? IMO this is more generalised approach over all.
I do this _VERY_ frequently in my databases, only I have my stored
procs do the aggregate in a predefined MVCC table that's always there.
Here's a denormalized version for public consumption/thought:
CREATE TABLE global.dba_aggregate_cache (
dbl TEXT NOT NULL, -- The database location, doesn't need to be
-- qualified (ex: schema.table.col)
op TEXT NOT NULL, -- The operation, SUM, COUNT, etc.
qual TEXT, -- Any kind of conditional, such as a where clause
val_int INT, -- Whatever the value is, of type INT
val_bigint BIGINT, -- Whatever the value is, of type BIGINT
val_text TEXT, -- Whatever the value is, of type TEXT
val_bytea BYTEA, -- Whatever the value is, of type BYTEA
);
CREATE UNIQUE INDEX dba_aggregate_cache_dbl_op_udx ON global.dba_aggregate_cache(dbl,op);
Then, I use a function to retrieve this value instead of a SELECT
COUNT(*).
SELECT public.cache_count('dbl','qual'); -- In this case, the op is COUNT
SELECT public.cache_count('dbl'); -- Returns the COUNT for the table listed in the dbl
Then, I create 4 or 5 functions (depends on the op I'm performing):
1) A private function that _doesn't_ run as security definer, that
populates the global.dba_aggregate_cache row if it's empty.
2) A STABLE function for SELECTs, if the row doesn't exist, then it
calls function #1 to populate its existence.
3) A STABLE function for INSERTs, if the row doesn't exist, then it
calls function #1 to populate its existence, then adds the
necessary bits to make it accurate.
4) A STABLE function for DELETEs, if the row doesn't exist, then it
calls function #1 to populate its existence, then deletes the
necessary bits to make it accurate.
5) A STABLE function for UPDATEs, if the row doesn't exist, then it
calls function #1 to populate its existence, then updates the
necessary bits to make it accurate. It's not uncommon for me to
not have an UPDATE function/trigger.
Create triggers for functions 2-5, and test away. It's MVCC,
searching through a table that's INDEX'ed for a single row is
obviously vastly faster than a seqscan/aggregate. If I need any kind
of an aggregate to be fast, I use this system with a derivation of the
above table. The problem with it being that I have to retrain others
to use cache_count(), or some other function instead of using
COUNT(*).
That said, it'd be nice if there were a way to tell PostgreSQL to do
the above for you and teach COUNT(*), SUM(*), or other aggregates to
use an MVCC backed cache similar to the above. If people want their
COUNT's to be fast, then they have to live with the INSERT, UPDATE,
DELETE cost. The above doesn't work with anything complex such as
join's, but it's certainly a start and I think satisfies everyone's
gripes other than the tuple churn that _does_ happen (*nudge nudge*,
pg_autovacuum could be integrated into the backend to handle this).
Those worried about performance, the pages that are constantly being
recycled would likely stay in disk cache (PG or the OS). There's
still some commit overhead, but still... no need to over optimize by
requiring the table to be stored in the out dated, slow, and over used
shm (also, *nudge nudge*).
Anyway, let me throw that out there as a solution that I use and it
works quite well. I didn't explain the use of the qual column, but I
think those who grasp the above way of handling things probably grok
how to use the qual column in a dynamically executed query.
CREATE AGGREGATE CACHE anyone?
-sc
--
Sean Chittenden
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