Re: [pgsql-patches] Recalculating OldestXmin in a long-running vacuum

Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
> I have two runs of DBT-2, one with the patch and one without.

> Patched:

> autovac "public.stock" scans:1 pages:1285990(-0)
> tuples:25303056(-2671265) CPU 95.22s/38.02u sec elapsed 10351.17 sec

> Unpatched:

> autovac "public.stock" scans:1 pages:1284504(-0)
> tuples:25001369(-1973760) CPU 86.55s/34.70u sec elapsed 9628.13 sec

So that makes this patch a good idea why? (Maybe what we need to see
is the impact on the total elapsed time for the DBT-2 test, rather
than just the VACUUM runtime.)

BTW I've got serious reservations about whether this bit is safe:

> + /* The table could've grown since vacuum started, and there
> + * might already be dead tuples on the new pages. Catch them
> + * as well. Also, we want to include any live tuples in the
> + * new pages in the statistics.
> + */
> + nblocks = RelationGetNumberOfBlocks(onerel);

I seem to recall some assumptions somewhere in the system that a vacuum
won't visit newly-added pages.

regards, tom lane