Re: pg_stat_statements: Avoid holding excessive lock

On Tue, Nov 05, 2024 at 08:37:08PM +0300, Karina Litskevich wrote:
> I suggest eliminating holding the excessive lock. See the attached patch.
> This would also restore the consistency between the code and the comments
> about entry's mutex spinlock usage.

You are right. minmax_stats_since and stats_since are only set when
an entry is allocated or reset, so this is not going to matter.

> + /*
> + * There is no need to hold entry->mutex when reading stats_since and
> + * minmax_stats_since for (unlike counters) they are always written
> + * while holding pgss->lock exclusively. We are holding pgss->lock
> + * shared so there should be no race here.
> + */
> stats_since = entry->stats_since;
> minmax_stats_since = entry->minmax_stats_since;
> - SpinLockRelease(&entry->mutex);

The comment could be simpler, say a "The spinlock is not required when
reading these two as they are always updated when holding pgss->lock
exclusively.". Or something like that.
--
Michael