Re: Need help understanding vacuum verbose output

Gordon Shannon <gordo169(at)gmail(dot)com> writes:
> Hi, Running 8.4.4 on Centos. A couple of these numbers don't make sense to
> me.
> (I added line numbers for reference)

> 11 INFO: "authors_archive": found 16558 removable, 7300 nonremovable row
> versions in 492 out of 51958 pages

The key point here is that vacuum only looked at 492 out of the 51958
pages in the table; those rowcounts are relevant to just that subset of
the table. This is a new thing in 8.4 --- before, vacuum always had to
scan the entire table. There's now bookkeeping that lets it figure out
which pages might contain vacuumable tuples, so it doesn't have to visit
every one. The only real downside is that this message doesn't tell the
whole truth :-(

(IIRC there isn't yet any comparable optimization on indexes, so the
reports about the indexes still reflect everything in the table.)

regards, tom lane