Tom Lane wrote:
> Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> > Hannu Krossing asked me about his patch to ignore transactions running
> > VACUUM LAZY in other vacuum transactions. I attach a version of the
> > patch updated to the current sources.
>
> nonInVacuumXmin seems useless ... perhaps a vestige of some earlier
> version of the computation?
Hmm ... I remember removing a now-useless variable somewhere, but maybe
this one escaped me. I don't have the code handy -- will check.
> In general, it seems to me that a transaction running lazy vacuum could
> be ignored for every purpose except truncating clog/subtrans. Since it
> will never insert its own XID into the database (note: VACUUM ANALYZE is
> run as two separate transactions, hence the pg_statistic rows inserted
> by ANALYZE are not a counterexample), there's no need for anyone to
> include it as running in their snapshots. So unless I'm missing
> something, this is a safe change for lazy vacuum, but perhaps not for
> full vacuum, which *does* put its XID into the database.
But keep in mind that in the current code, clog truncation takes
relminxid (actually datminxid) into account, not running transactions,
so AFAICS this should affect anything.
Subtrans truncation is different and it certainly should consider lazy
vacuum's Xids.
> A possible objection to this is that it would foreclose running VACUUM
> and ANALYZE as a single transaction, exactly because of the point that
> we couldn't insert pg_statistic rows using a lazy vacuum's XID. I think
> there was some discussion of doing that in connection with enlarging
> ANALYZE's sample greatly --- if ANALYZE goes back to being a full scan
> or nearly so, it'd sure be nice to combine it with the VACUUM scan.
> However maybe we should just accept that as the price of not having
> multiple vacuums interfere with each other.
Hmm, what about having a single scan for both, and then starting a
normal transaction just for the sake of inserting the pg_statistics
tuple?
I think the interactions of Xids and vacuum and other stuff are starting
to get complex; IMHO it warrants having a README.vacuum, or something.
--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.