Re: pg_autovacuum not having enough suction ?

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Tom Lane wrote:
> >> I'm not sure if autovacuum could be taught to do that --- it could
> >> perhaps launch a vacuum as soon as it notices a large fraction of the
> >> table got deleted, but do we really want to authorize it to launch
> >> VACUUM FULL?
>
> > One problem with VACUUM FULL would be autovacuum waiting for an
> > exclusive lock on the table.  Anyway, it is documented now as a possible
> > issue.
>
> I don't care too much about autovacuum waiting awhile to get a lock.
> I do care about other processes getting queued up behind it, though.
>
> Perhaps it would be possible to alter the normal lock queuing semantics
> for this case, so that autovacuum's request doesn't block later
> arrivals, and it can only get the lock when no one is interested in the
> table.  Of course, that might never happen, or by the time it does
> there's no point in VACUUM FULL anymore :-(

Can we issue a LOCK TABLE with a statement_timeout, and only do the
VACUUM FULL if we can get a lock quickly?  That seems like a plan.

The only problem is that you can't VACUUM FULL in a transaction:

    test=> create table test (x int);
    CREATE TABLE
    test=> insert into test values (1);
    INSERT 0 1
    test=> begin;
    BEGIN
    test=> lock table test;
    LOCK TABLE
    test=> vacuum full;
    ERROR:  VACUUM cannot run inside a transaction block

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