Re: Option to not use ringbuffer in VACUUM, using it in failsafe mode

Hi,

I was just doing some cleanup on the main patch in this set and realized
that it was missing a few things. One of which is forbidding the
BUFFER_USAGE_LIMIT with VACUUM FULL since VACUUM FULL does not use a
BAS_VACUUM strategy.

VACUUM FULL technically uses a bulkread buffer access strategy for
reading the original relation if its number of blocks is > number of
shared buffers / 4 (see initscan()). The new rel writing is done using
smgrextend/write directly and doesn't go through shared buffers. I
think it is a stretch to try and use the size passed in to VACUUM by
BUFFER_USAGE_LIMIT for the bulkread strategy ring.

As for forbidding the combination, I noticed that when VACUUM FULL is
specified with INDEX_CLEANUP OFF, there is no syntax error but the
INDEX_CLEANUP option is simply ignored. This is documented behavior.

I somehow feel like VACUUM (FULL, BUFFER_USAGE_LIMIT 'x') should error
out instead of silently not using the buffer usage limit, though.

I am looking for others' opinions.

- Melanie