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postgresql/doc/TODO.detail/exists
Bruce Momjian 1a712718ca Add IN/EXISTS file.
2002-01-20 05:12:57 +00:00

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From pgsql-sql-owner+M5999=candle.pha.pa.us=pgman@postgresql.org Mon Dec 17 01:39:56 2001
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To: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
cc: "Stephan Szabo" <sszabo@megazone23.bigpanda.com>,
"MindTerm" <mindterm@yahoo.com>, pgsql-sql@postgresql.org
Subject: Re: [SQL] performance tuning in large function / transaction
In-Reply-To: <GNELIHDDFBOCMGBFGEFOIENDCAAA.chriskl@familyhealth.com.au>
References: <GNELIHDDFBOCMGBFGEFOIENDCAAA.chriskl@familyhealth.com.au>
Comments: In-reply-to "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
message dated "Mon, 17 Dec 2001 12:06:14 +0800"
Date: Mon, 17 Dec 2001 01:21:16 -0500
Message-ID: <29547.1008570076@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
> Is it true that the IN command is implemented sort of as a linked list
> linear time search? Is there any plan for a super-fast implementation of
> 'IN'?
This deserves a somewhat long-winded answer.
Postgres presently supports two kinds of IN (I'm not sure whether SQL92
allows any additional kinds):
1. Scalar-list IN: foo IN ('bar', 'baz', 'quux', ...)
2. Sub-select IN: foo IN (SELECT bar FROM ...)
In the scalar-list form, a variable is compared to an explicit list of
constants or expressions. This form is exactly equivalent to
foo = 'bar' OR foo = 'baz' OR foo = 'quux' OR ...
and is converted into that form by the parser. The planner is capable
of converting a WHERE clause of this kind into multiple passes of
indexscan, when foo is an indexed column and all the IN-list elements
are constants. Whether it actually will make that conversion depends
on the usual vagaries of pg_statistic entries, etc. But if it's a
unique or fairly-selective index, and there aren't a huge number of
entries in the IN list, a multiple indexscan should be a good plan.
In the sub-select form, we pretty much suck: for each tuple in the outer
query, we run the inner query until we find a matching value or the
inner query ends. This is basically a nested-loop scenario, with the
only (minimally) redeeming social value being that the planner realizes
it should pick a fast-start plan for the inner query. I think it should
be possible to convert this form into a modified kind of join (sort of
the reverse of an outer join: rather than at least one result per
lefthand row, at most one result per lefthand row), and then we could
use join methods that are more efficient than nested-loop. But no one's
tried to make that happen yet.
regards, tom lane
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From: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
To: "Tom Lane" <tgl@sss.pgh.pa.us>
cc: "Stephan Szabo" <sszabo@megazone23.bigpanda.com>,
"MindTerm" <mindterm@yahoo.com>, <pgsql-sql@postgresql.org>
Subject: [SQL] 'IN' performance
Date: Mon, 17 Dec 2001 14:33:40 +0800
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> In the sub-select form, we pretty much suck: for each tuple in the outer
> query, we run the inner query until we find a matching value or the
> inner query ends. This is basically a nested-loop scenario, with the
> only (minimally) redeeming social value being that the planner realizes
> it should pick a fast-start plan for the inner query. I think it should
> be possible to convert this form into a modified kind of join (sort of
> the reverse of an outer join: rather than at least one result per
> lefthand row, at most one result per lefthand row), and then we could
> use join methods that are more efficient than nested-loop. But no one's
> tried to make that happen yet.
That's what I was thinking...where abouts does all that activity happen?
I assume the planner knows that it doesn't have to reevaluate the subquery
if it's not correlated?
Chris
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From pgsql-sql-owner+M6001=candle.pha.pa.us=pgman@postgresql.org Mon Dec 17 02:00:10 2001
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To: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
cc: "Stephan Szabo" <sszabo@megazone23.bigpanda.com>,
"MindTerm" <mindterm@yahoo.com>, pgsql-sql@postgresql.org
Subject: Re: [SQL] 'IN' performance
In-Reply-To: <GNELIHDDFBOCMGBFGEFOEENFCAAA.chriskl@familyhealth.com.au>
References: <GNELIHDDFBOCMGBFGEFOEENFCAAA.chriskl@familyhealth.com.au>
Comments: In-reply-to "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
message dated "Mon, 17 Dec 2001 14:33:40 +0800"
Date: Mon, 17 Dec 2001 01:44:03 -0500
Message-ID: <29730.1008571443@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-sql-owner@postgresql.org
Status: OR
"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
> That's what I was thinking...where abouts does all that activity happen?
The infrastructure for different join rules already exists. There'd
need to be a new JOIN_xxx type added to the various join nodes in the
executor, but AFAICS that's just a minor extension. The part that is
perhaps not trivial is in the planner. All the existing inner and outer
join types start out expressed as joins in the original query. To make
IN into a join, the planner would have to hoist up a clause from WHERE
into the join-tree structure. I think it can be done, but I have not
thought hard about where and how, nor about what semantic restrictions
might need to be checked.
regards, tom lane
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