From: Nathan Bossart <nathandbossart@gmail.com>
To: Sami Imseih <samimseih@gmail.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alexander Lakhin <exclusion@gmail.com>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Cc: Robert Treat <rob@xzilla.net>
Cc: satyanarlapuram@gmail.com, pgsql-hackers <pgsql-hackers@postgresql.org>
Cc: tndrwang@gmail.com
Subject: Re: Add pg_stat_autovacuum_priority
Date: Wed, 8 Apr 2026 16:48:31 -0500
Message-ID: <adbNL1puSbBi8nMo@nathan> (raw)
In-Reply-To: <CAA5RZ0uL1ukKvV0Gwgok9ut_203q_E_tzBrwhDAuJCKVNwWZVw@mail.gmail.com>
References: <adQsdvPPNviWMCXb@nathan>
<5684f479-858e-4c5d-b8f5-bcf05de1f909@gmail.com>
<3077290.1775668665@sss.pgh.pa.us>
<adaWuTR7oCKodH7k@nathan>
<3097984.1775674400@sss.pgh.pa.us>
<adan_E0o69p81lIj@nathan>
<3101163.1775676098@sss.pgh.pa.us>
<u3akkfw3vaocviudt6f7ft4kxjc2273rh3tfhz5rfwg6jrcc2t@wurwws6wgjjz>
<adbHYQMZ0pHQttWa@nathan>
<CAA5RZ0uL1ukKvV0Gwgok9ut_203q_E_tzBrwhDAuJCKVNwWZVw@mail.gmail.com>
On Wed, Apr 08, 2026 at 04:33:00PM -0500, Sami Imseih wrote:
>> On Wed, Apr 08, 2026 at 04:40:03PM -0400, Andres Freund wrote:
>> > Note that the whole cached state does automatically get reset at the end of
>> > the transaction (AtEOXact_PgStat()->pgstat_clear_snapshot()), just like it did
>> > before the shmem stats stuff.
>>
>> I see a lot of memory used for the pgStatEntryRefHash table, too (e.g., ~16
>> MB for 100K tables). What's interesting is that I cannot reproduce similar
>> usage with views like pg_stat_all_tables. If memory was not a concern, I
>> think the "bool *may_free" idea would be fine.
>
> Instead of may_free, which is invasive, what about pgstat_fetch_entry_nocache
> which can be called by 2 new APIs pgstat_fetch_stat_tabentry_nocache() and
> pgstat_fetch_stat_tabentry_nocache_ext(). This way a caller that uses
> these will be required to pfree?
This might help avoid memory usage within a snapshot, but as Andres notes,
this gets released automatically at the end of the transaction.
For a database with 100K tables, here's what I see before calling the view:
name | used_bytes
------------------------+------------
PgStat Shared Ref Hash | 8576
PgStat Shared Ref | 2960
PgStat Pending | 4712
(3 rows)
After calling the view but before committing the transaction, I see this:
name | used_bytes
------------------------+------------
PgStat Snapshot | 4194688
PgStat Shared Ref Hash | 4194688
PgStat Shared Ref | 8048240
PgStat Pending | 3064
(4 rows)
And after committing, I see:
name | used_bytes
------------------------+------------
PgStat Shared Ref Hash | 4194688
PgStat Shared Ref | 8048240
PgStat Pending | 360
(3 rows)
--
nathan
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Cc: nathandbossart@gmail.com, samimseih@gmail.com, andres@anarazel.de, tgl@sss.pgh.pa.us, exclusion@gmail.com, bharath.rupireddyforpostgres@gmail.com, rob@xzilla.net, tndrwang@gmail.com
Subject: Re: Add pg_stat_autovacuum_priority
In-Reply-To: <adbNL1puSbBi8nMo@nathan>
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