Message-ID: <d7e2f7606818c629dbc684575a6c32a2045e0209.camel@j-davis.com>
Subject: Re: Significant performance issues with array_agg() + HashAggregate
 plans on Postgres 17
From: Jeff Davis <pgsql@j-davis.com>
To: Tom Lane <tgl@sss.pgh.pa.us>, Scott Carey <scott.carey@algonomy.com>
Cc: David Rowley <dgrowleyml@gmail.com>,
	pgsql-performance@lists.postgresql.org
Date: Fri, 03 Apr 2026 12:24:49 -0700
In-Reply-To: <3671092.1775167393@sss.pgh.pa.us>
References: 
	<CA+vubOGaanYaGGb98etJPeJCbN=RAawx652MdmJVRK8+BeEP0w@mail.gmail.com>
	 <CAApHDvq5fMzhK7OwGOhOgqRF-wErGNsT75yie5LtobSir1HhnA@mail.gmail.com>
	 <CA+vubOE6feEWh7XxoWqdPh634SUVjHKW7s9BgMD755wJTqQFPw@mail.gmail.com>
	 <CA+vubOHgrdkm+3wgC0FPcStDeMS9ZQ_Du9vUXWjxN9MN9CO2uw@mail.gmail.com>
	 <2764544.1774985169@sss.pgh.pa.us>
	 <CA+vubOEbfm5+BxtuFGb_tTj3sSX1u0Y+=zHUxoF1Q9feGpRfNg@mail.gmail.com>
	 <2897039.1775051081@sss.pgh.pa.us>
	 <CA+vubOGqg7f2E0_OW8EMruAObejnCS1gCgguT+cnwHG9bRxMDQ@mail.gmail.com>
	 <3579239.1775151498@sss.pgh.pa.us>
	 <CA+vubOH1mcEbsHnkm5L7aJWR+eeiVpOnGfcdWax9td6pyOkMww@mail.gmail.com>
	 <3671092.1775167393@sss.pgh.pa.us>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
User-Agent: Evolution 3.52.3-0ubuntu1.1 
MIME-Version: 1.0
Archived-At: 
 <https://www.postgresql.org/message-id/d7e2f7606818c629dbc684575a6c32a2045e0209.camel%40j-davis.com>
Precedence: bulk

On Thu, 2026-04-02 at 18:03 -0400, Tom Lane wrote:

> and a secondary problem is that that drives the
> cost of hash_agg_check_limits to an unacceptable level.

I recall some discussion about whether the memory accounting would
recurse to child contexts at the time MemoryContextGetMemAllocated() is
called, or whether it would update the parent contexts at the time a
new block is allocated in a subcontext. Using the latter strategy would
solve the high cost when there are many subcontexts.

Regards,
	Jeff Davis