pg.ddx.io  pgsql-performance@postgresql.org mailing list archive  

help / color / mirror / Atom feed
From: Tom Lane <tgl@sss.pgh.pa.us>
To: ldh@laurent-hasson.com <ldh@laurent-hasson.com>
Cc: Peter Geoghegan <pg@bowt.ie>
Cc: David Rowley <dgrowleyml@gmail.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: Tue, 27 Jul 2021 23:31:17 -0400
Message-ID: <544431.1627443077@sss.pgh.pa.us> (raw)
In-Reply-To: <543673.1627442152@sss.pgh.pa.us>
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>
	<787780.1626972122@sss.pgh.pa.us>
	<MN2PR15MB25609F3505A6FB441CAB22A585E49@MN2PR15MB2560.namprd15.prod.outlook.com>
	<171155.1627236507@sss.pgh.pa.us>
	<MN2PR15MB2560987D103C712238C706AB85EA9@MN2PR15MB2560.namprd15.prod.outlook.com>
	<543673.1627442152@sss.pgh.pa.us>

I wrote:
> Yeah, I wouldn't sweat over the specific value.  The pre-v13 behavior
> was effectively equivalent to hash_mem_multiplier = infinity, so if
> you weren't having any OOM problems before, just crank it up.

Oh, wait, scratch that: the old executor's behavior is accurately
described by that statement, but the planner's is not.  The planner
will not pick a hashagg plan if it guesses that the hash table
would exceed the configured limit (work_mem before, now work_mem
times hash_mem_multiplier).  So raising hash_mem_multiplier to the
moon might bias the v13 planner to pick hashagg plans in cases
where earlier versions would not have.  This doesn't describe your
immediate problem, but it might be a reason to not just set the
value as high as you can.

BTW, this also suggests that the planner is underestimating the
amount of memory needed for the hashagg, both before and after.
That might be something to investigate at some point.

			regards, tom lane





view thread (59+ messages)  latest in thread

Message-ID: <544431.1627443077@sss.pgh.pa.us>
Permalink:  ../544431.1627443077@sss.pgh.pa.us/
Also on:    postgresql.org/message-id/544431.1627443077@sss.pgh.pa.us

 · 

reply
Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: pgsql-performance@postgresql.org
  Cc: tgl@sss.pgh.pa.us, ldh@laurent-hasson.com, pg@bowt.ie, dgrowleyml@gmail.com, pryzby@telsasoft.com
  Subject: Re: Big performance slowdown from 11.2 to 13.3
  In-Reply-To: <544431.1627443077@sss.pgh.pa.us>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox