From: Tom Lane <tgl@sss.pgh.pa.us>
To: Floris Van Nee <florisvannee@Optiver.com>
cc: "pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>,
        Andres Freund <andres@anarazel.de>
Subject: Re: error "can only drop stats once" brings down database
In-reply-to: <8e5cb534706a4f43bd74405c8fd91437@Optiver.com>
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 <8e5cb534706a4f43bd74405c8fd91437@Optiver.com>
Comments: In-reply-to Floris Van Nee <florisvannee@Optiver.com>
	message dated "Fri, 03 May 2024 18:10:05 -0000"
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Date: Fri, 03 May 2024 15:32:26 -0400
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Floris Van Nee <florisvannee@Optiver.com> writes:
>> Hmm.  This must be coming from pgstat_drop_entry_internal.
>> I suspect the correct fix is in pgstat_drop_entry, along the lines of
>> 
>> -	if (shent)
>> +	if (shent && !shent->dropped)
>> 
>> but it's not clear to me how the already-dropped case ought to affect the
>> function's bool result.  Also, how are we getting into a concurrent-drop
>> situation in recovery?

> Anyone has further thoughts on this? This still happens occasionally.

I was expecting Andres to comment on it eventually.  I don't know
that code well enough to want to modify it on my own authority,
especially not just before a release.

One thing that would be safe enough and perhaps useful is to
modify the "can only drop stats once" message to provide a
little more detail, like the hash key of the problematic
entry.  That might help us understand what's triggering this.
The OIDs would be opaque perhaps, but the PgStat_Kind value
seems useful to know.

			regards, tom lane