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From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
Cc: ldh@laurent-hasson.com <ldh@laurent-hasson.com>
Cc: Peter Geoghegan <pg@bowt.ie>
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 12:21:38 -0400
Message-ID: <786600.1626970898@sss.pgh.pa.us> (raw)
In-Reply-To: <CAApHDvo+Gb80BVPKYCwHbLa7a7ZXp9ziq9URrpQaKdvXsd46bg@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>

David Rowley <dgrowleyml@gmail.com> writes:
> On Fri, 23 Jul 2021 at 03:56, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> So basically, we now have a hard restriction that hashaggs can't use
>> more than INT_MAX kilobytes, or approximately 2.5GB, and this use case
>> is getting eaten alive by that restriction.  Seems like we need to
>> do something about that.

> Hmm, math check?

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.

I think the right fix here is to remove the cap, which will require
changing get_hash_mem to return double, and then maybe some cascading
changes --- I've not looked at its callers.

			regards, tom lane





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