Message-ID: <1836967a-9f78-aed8-1ff1-3351c504e8f0@gmail.com>
Date: Fri, 24 Feb 2023 19:00:00 +0300
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Subject: Re: BUG #17804: Assertion failed in pg_stat after fetching from
 pg_stat_database and switching cache->snapshot
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To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
 pgsql-bugs@lists.postgresql.org
References: <17804-2a118cd046f2d0e5@postgresql.org>
 <20230224.160812.1001813717917457753.horikyota.ntt@gmail.com>
From: Alexander Lakhin <exclusion@gmail.com>
In-Reply-To: <20230224.160812.1001813717917457753.horikyota.ntt@gmail.com>
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Hello Kyotaro-san,
24.02.2023 10:08, Kyotaro Horiguchi wrote:
> Thanks for the report!  We didn't consider the case that variable is
> changed in-transaction.
>
>
> What are your thoughts on this?
What if we implement assign_pgstat_fetch_consistency() and execute
pgstat_clear_snapshot() in it when the mode switch happens (to avoid
possible inconsistency between the GUC and the current pgstat state)?
If snapshots in different modes contain different data sets, will such
behaviour be unexpected for a user?
For example, if a user performs:
SET LOCAL stats_fetch_consistency = snapshot;
{some-actions-1}
SET LOCAL stats_fetch_consistency = none;
{some-actions-2}
SET LOCAL stats_fetch_consistency = snapshot;
{some-actions-3}
Should she expect that some-actions-1, some-actions-2 will be
reflected in the snapshot at the end?

Best regards,
Alexander