Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On 2016-05-24 13:04:09 -0500, Kevin Grittner wrote:
> On Tue, May 24, 2016 at 12:00 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> > Analyze IIRC acquires a new snapshot when getting sample rows,
>
> I could not find anything like that, and a case-insensitive search
> of analyze.c finds no occurrences of "snap". Can you remember
> where you think you saw something that would cause the ANALYZE
> command in my test to use a snapshot other than the one from the
> REPEATABLE READ transaction in which it was run?

It's outside of analyze.c:

autovacuum_do_vac_analyze() -> vacuum() ->
if (options & VACOPT_VACUUM)
use_own_xacts = true;
else
{
Assert(options & VACOPT_ANALYZE);
if (IsAutoVacuumWorkerProcess())
use_own_xacts = true;
...
if (options & VACOPT_ANALYZE)
{
/*
* If using separate xacts, start one for analyze. Otherwise,
* we can use the outer transaction.
*/
if (use_own_xacts)
{
StartTransactionCommand();
/* functions in indexes may want a snapshot set */
PushActiveSnapshot(GetTransactionSnapshot());
}

analyze_rel(relid, relation, options, params,
va_cols, in_outer_xact, vac_strategy);

if (use_own_xacts)
{
PopActiveSnapshot();
CommitTransactionCommand();
}
}

Andres