VACUUM FULL/CLUSTER doesn't update pg_class's pg_class.relfrozenxid

Hi,

As Robert previously complained a database wide VACUUM FULL now (as of
3cff1879f8d03) reliably increases the relfrozenxid for all tables but
pg_class itself. That's a bit sad because it means doing a VACUUM FULL
won't help in a anti-wraparound scenario.

The reason for that is explained by the following comment:
/*
* Update the tuples in pg_class --- unless the target relation of the
* swap is pg_class itself. In that case, there is zero point in making
* changes because we'd be updating the old data that we're about to throw
* away. Because the real work being done here for a mapped relation is
* just to change the relation map settings, it's all right to not update
* the pg_class rows in this case.
*/

I think the easiest fix for that is to update pg_class' relfrozenxid in
finish_heap_swap() after the indexes have been rebuilt, that's just a
couple of lines. There's more complex solutions that'd avoid the need
for that special case, but I it's sufficient. A patch doing that is
attached.

Note that VACUUM FULL will still require more xids than a plain VACUUM,
but it scales linearly with the number of relations, so I have a hard
time seing that as problematic.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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