77 lines
3.2 KiB
Plaintext
77 lines
3.2 KiB
Plaintext
From vadim@krs.ru Fri Aug 6 00:02:02 1999
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Received: from sunpine.krs.ru (SunPine.krs.ru [195.161.16.37])
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for <maillist@candle.pha.pa.us>; Fri, 6 Aug 1999 00:02:00 -0400 (EDT)
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Fri, 6 Aug 1999 12:01:59 +0800 (KRSS)
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Message-ID: <37AA5E35.66C03F2E@krs.ru>
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Date: Fri, 06 Aug 1999 12:01:57 +0800
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From: Vadim Mikheev <vadim@krs.ru>
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Organization: OJSC Rostelecom (Krasnoyarsk)
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X-Mailer: Mozilla 4.5 [en] (X11; I; FreeBSD 3.0-RELEASE i386)
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MIME-Version: 1.0
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To: Bruce Momjian <maillist@candle.pha.pa.us>
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CC: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@postgreSQL.org
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Subject: Re: [HACKERS] Idea for speeding up uncorrelated subqueries
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References: <199908060331.XAA22277@candle.pha.pa.us>
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Content-Type: text/plain; charset=us-ascii
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Content-Transfer-Encoding: 7bit
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Status: RO
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Bruce Momjian wrote:
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>
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> Isn't it something that takes only a few hours to implement. We can't
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> keep telling people to us EXISTS, especially because most SQL people
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> think correlated queries are slower that non-correlated ones. Can we
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> just on-the-fly rewrite the query to use exists?
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This seems easy to implement. We could look does subquery have
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aggregates or not before calling union_planner() in
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subselect.c:_make_subplan() and rewrite it (change
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slink->subLinkType from IN to EXISTS and add quals).
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Without caching implemented IN-->EXISTS rewriting always
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has sence.
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After implementation of caching we probably should call union_planner()
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for both original/modified subqueries and compare costs/sizes
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of EXISTS/IN_with_caching plans and maybe even make
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decision what plan to use after parent query is planned
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and we know for how many parent rows subplan will be executed.
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Vadim
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From tgl@sss.pgh.pa.us Fri Aug 6 00:15:23 1999
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Received: from sss.sss.pgh.pa.us (sss.pgh.pa.us [209.114.166.2])
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for <maillist@candle.pha.pa.us>; Fri, 6 Aug 1999 00:15:22 -0400 (EDT)
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by sss.sss.pgh.pa.us (8.9.1/8.9.1) with ESMTP id AAA06786;
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Fri, 6 Aug 1999 00:14:50 -0400 (EDT)
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To: Bruce Momjian <maillist@candle.pha.pa.us>
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cc: Vadim Mikheev <vadim@krs.ru>, pgsql-hackers@postgreSQL.org
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Subject: Re: [HACKERS] Idea for speeding up uncorrelated subqueries
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In-reply-to: Your message of Thu, 5 Aug 1999 23:31:01 -0400 (EDT)
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<199908060331.XAA22277@candle.pha.pa.us>
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Date: Fri, 06 Aug 1999 00:14:50 -0400
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Message-ID: <6783.933912890@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Status: RO
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Bruce Momjian <maillist@candle.pha.pa.us> writes:
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> Isn't it something that takes only a few hours to implement. We can't
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> keep telling people to us EXISTS, especially because most SQL people
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> think correlated queries are slower that non-correlated ones. Can we
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> just on-the-fly rewrite the query to use exists?
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I was just about to suggest exactly that. The "IN (subselect)"
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notation seems to be a lot more intuitive --- at least, people
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keep coming up with it --- so why not rewrite it to the EXISTS
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form, if we can handle that more efficiently?
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regards, tom lane
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