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@anarazel.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