Feedback-ID: i0fe9450f:Fastmail
Date: Sun, 16 Aug 2026 08:32:16 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Konstantin Knizhnik <knizhnik@garret.ru>,
	Heikki Linnakangas <hlinnaka@iki.fi>,
	PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Subject: Re: SIGSEGV in dynahash
Message-ID: <aoD3ACTRYXaF7t1-@paquier.xyz>
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On Sat, Aug 15, 2026 at 12:12:29PM -0400, Tom Lane wrote:
> Yeah.  I'm not too pleased with 9fe9ecd516b for a different reason.
>=20
> In pursuit of what seems to be a merely cosmetic goal (ie make shared
> hashes be reported differently in pg_shmem_allocations), it's made a
> fundamental and IMO possibly destabilizing change in the behavior of
> shared-memory hash tables. To wit, it is no longer possible to expand
> a shared hash table beyond its startup-time allocation.

There is a difference between shmem_hash.c and dynahash.c.  dynahash.c
is able to support a growing size, but it's not the case of
shmem_hash.c.

Note also some comments in dynahash.c:
 * table grows much beyond the initial size.  (Currently, shared memory hash
 * tables are only created by ShmemRequestHash()/ShmemInitHash() though, wh=
ich
 * doesn't support growing at all.)=20

shmem_hash_create() also claims the non-growth argument as true as we
would need to grow the underlying shmem region linked with a hash
table defined by shmem_hash.c.

Or am I missing something?
--
Michael

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