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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Geoghegan <pg@bowt.ie>
Cc: David Rowley <dgrowleyml@gmail.com>
Cc: ldh@laurent-hasson.com <ldh@laurent-hasson.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Big performance slowdown from 11.2 to 13.3
Date: Thu, 22 Jul 2021 12:42:02 -0400
Message-ID: <787780.1626972122@sss.pgh.pa.us> (raw)
In-Reply-To: <CAH2-WzmftijCna_aY-HyCUfswwfPCC4d2HjKab1cxM0QCFrRUQ@mail.gmail.com>
References: <MN2PR15MB25601E80A9B6D1BA6F592B1985E39@MN2PR15MB2560.namprd15.prod.outlook.com>
	<20210721191528.GC19620@telsasoft.com>
	<MN2PR15MB256016E33F00DE60B96F02D385E39@MN2PR15MB2560.namprd15.prod.outlook.com>
	<CAH2-Wzm3rSAWHyENb3T0X8M_qdYjF1t7rntkMJ2kDiuhVOHx3g@mail.gmail.com>
	<MN2PR15MB2560C1A890DC07F9359FB56285E39@MN2PR15MB2560.namprd15.prod.outlook.com>
	<732758.1626911007@sss.pgh.pa.us>
	<MN2PR15MB2560778595BB7AF6E691B26785E39@MN2PR15MB2560.namprd15.prod.outlook.com>
	<MN2PR15MB2560D429866BD65A70574A6C85E49@MN2PR15MB2560.namprd15.prod.outlook.com>
	<CAApHDvqoF7y7SQjGT=940njnAd_CUcUG6oRYChcP_+eJyuNiug@mail.gmail.com>
	<MN2PR15MB25603D3CE06C8F9C312EA58985E49@MN2PR15MB2560.namprd15.prod.outlook.com>
	<MN2PR15MB2560E4574A6E2B5AED2C03CB85E49@MN2PR15MB2560.namprd15.prod.outlook.com>
	<784576.1626968702@sss.pgh.pa.us>
	<785218.1626969409@sss.pgh.pa.us>
	<CAApHDvo+Gb80BVPKYCwHbLa7a7ZXp9ziq9URrpQaKdvXsd46bg@mail.gmail.com>
	<786600.1626970898@sss.pgh.pa.us>
	<CAH2-WzmftijCna_aY-HyCUfsww! fPCC4d2HjKab1cxM0QCFrRUQ@mail.gmail.com>

Peter Geoghegan <pg@bowt.ie> writes:
> On Thu, Jul 22, 2021 at 9:21 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Yeah, I should have said "2GB plus palloc slop".  It doesn't surprise
>> me a bit that we seem to be eating another 20% on top of the nominal
>> limit.

> MAX_KILOBYTES is the max_val for the work_mem GUC itself, and has been
> for many years.

Right.  The point here is that before v13, hash aggregation was not
subject to the work_mem limit, nor any related limit.  If you did an
aggregation requiring more than 2GB-plus-slop, it would work just fine
as long as your machine had enough RAM.  Now, the performance sucks and
there is no knob you can turn to fix it.  That's unacceptable in my book.

			regards, tom lane





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