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192 lines
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192 lines
9.6 KiB
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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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Mon, 17 Dec 2001 01:21:17 -0500 (EST)
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To: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
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cc: "Stephan Szabo" <sszabo@megazone23.bigpanda.com>,
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"MindTerm" <mindterm@yahoo.com>, pgsql-sql@postgresql.org
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Subject: Re: [SQL] performance tuning in large function / transaction
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In-Reply-To: <GNELIHDDFBOCMGBFGEFOIENDCAAA.chriskl@familyhealth.com.au>
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References: <GNELIHDDFBOCMGBFGEFOIENDCAAA.chriskl@familyhealth.com.au>
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Comments: In-reply-to "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
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message dated "Mon, 17 Dec 2001 12:06:14 +0800"
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Date: Mon, 17 Dec 2001 01:21:16 -0500
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Message-ID: <29547.1008570076@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Precedence: bulk
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Sender: pgsql-sql-owner@postgresql.org
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Status: OR
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"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
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> Is it true that the IN command is implemented sort of as a linked list
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> linear time search? Is there any plan for a super-fast implementation of
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> 'IN'?
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This deserves a somewhat long-winded answer.
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Postgres presently supports two kinds of IN (I'm not sure whether SQL92
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allows any additional kinds):
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1. Scalar-list IN: foo IN ('bar', 'baz', 'quux', ...)
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2. Sub-select IN: foo IN (SELECT bar FROM ...)
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In the scalar-list form, a variable is compared to an explicit list of
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constants or expressions. This form is exactly equivalent to
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foo = 'bar' OR foo = 'baz' OR foo = 'quux' OR ...
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and is converted into that form by the parser. The planner is capable
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of converting a WHERE clause of this kind into multiple passes of
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indexscan, when foo is an indexed column and all the IN-list elements
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are constants. Whether it actually will make that conversion depends
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on the usual vagaries of pg_statistic entries, etc. But if it's a
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unique or fairly-selective index, and there aren't a huge number of
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entries in the IN list, a multiple indexscan should be a good plan.
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In the sub-select form, we pretty much suck: for each tuple in the outer
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query, we run the inner query until we find a matching value or the
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inner query ends. This is basically a nested-loop scenario, with the
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only (minimally) redeeming social value being that the planner realizes
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it should pick a fast-start plan for the inner query. I think it should
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be possible to convert this form into a modified kind of join (sort of
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the reverse of an outer join: rather than at least one result per
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lefthand row, at most one result per lefthand row), and then we could
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use join methods that are more efficient than nested-loop. But no one's
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tried to make that happen yet.
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regards, tom lane
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From pgsql-sql-owner+M6000=candle.pha.pa.us=pgman@postgresql.org Mon Dec 17 01:49:56 2001
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Return-path: <pgsql-sql-owner+M6000=candle.pha.pa.us=pgman@postgresql.org>
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Mon, 17 Dec 2001 14:33:07 +0800 (WST)
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From: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
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To: "Tom Lane" <tgl@sss.pgh.pa.us>
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cc: "Stephan Szabo" <sszabo@megazone23.bigpanda.com>,
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"MindTerm" <mindterm@yahoo.com>, <pgsql-sql@postgresql.org>
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Subject: [SQL] 'IN' performance
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Date: Mon, 17 Dec 2001 14:33:40 +0800
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Precedence: bulk
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Sender: pgsql-sql-owner@postgresql.org
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Status: OR
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> In the sub-select form, we pretty much suck: for each tuple in the outer
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> query, we run the inner query until we find a matching value or the
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> inner query ends. This is basically a nested-loop scenario, with the
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> only (minimally) redeeming social value being that the planner realizes
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> it should pick a fast-start plan for the inner query. I think it should
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> be possible to convert this form into a modified kind of join (sort of
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> the reverse of an outer join: rather than at least one result per
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> lefthand row, at most one result per lefthand row), and then we could
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> use join methods that are more efficient than nested-loop. But no one's
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> tried to make that happen yet.
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That's what I was thinking...where abouts does all that activity happen?
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I assume the planner knows that it doesn't have to reevaluate the subquery
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if it's not correlated?
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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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Return-path: <pgsql-sql-owner+M6001=candle.pha.pa.us=pgman@postgresql.org>
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Mon, 17 Dec 2001 01:44:03 -0500 (EST)
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To: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
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cc: "Stephan Szabo" <sszabo@megazone23.bigpanda.com>,
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"MindTerm" <mindterm@yahoo.com>, pgsql-sql@postgresql.org
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Subject: Re: [SQL] 'IN' performance
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In-Reply-To: <GNELIHDDFBOCMGBFGEFOEENFCAAA.chriskl@familyhealth.com.au>
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References: <GNELIHDDFBOCMGBFGEFOEENFCAAA.chriskl@familyhealth.com.au>
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Comments: In-reply-to "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
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message dated "Mon, 17 Dec 2001 14:33:40 +0800"
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Date: Mon, 17 Dec 2001 01:44:03 -0500
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Message-ID: <29730.1008571443@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Precedence: bulk
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Sender: pgsql-sql-owner@postgresql.org
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Status: OR
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"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
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> That's what I was thinking...where abouts does all that activity happen?
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The infrastructure for different join rules already exists. There'd
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need to be a new JOIN_xxx type added to the various join nodes in the
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executor, but AFAICS that's just a minor extension. The part that is
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perhaps not trivial is in the planner. All the existing inner and outer
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join types start out expressed as joins in the original query. To make
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IN into a join, the planner would have to hoist up a clause from WHERE
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into the join-tree structure. I think it can be done, but I have not
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thought hard about where and how, nor about what semantic restrictions
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might need to be checked.
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regards, tom lane
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---------------------------(end of broadcast)---------------------------
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TIP 2: you can get off all lists at once with the unregister command
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(send "unregister YourEmailAddressHere" to majordomo@postgresql.org)
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