Thread: Trying to understand VACUUM VERBOSE

Quickie questions:

When VACUUM VERBOSE says a table has x pages, what is the size of
the indicated page?

I'm trying to correlate table size with the output of an EXPLAIN
of the SELECT INTO which created it.

I've got 33150 rows which the EXPLAIN said at top level would be
80-byte rows.  VACUUM indicates 488 pages.  Does this compute?

33150 x 80 = 2,652,000
2652000/488 = 5434

Is it reasonable to assume some overhead and that the pages are
8k?  Is this why there's an 8k row-size limit?

++ kevin


--
Kevin O'Gorman  (805) 650-6274  mailto:kogorman@pacbell.net
Permanent e-mail forwarder:  mailto:Kevin.O'Gorman.64@Alum.Dartmouth.org
At school: mailto:kogorman@cs.ucsb.edu
Web: http://www.cs.ucsb.edu/~kogorman/index.html
Web: http://trixie.kosman.via.ayuda.com/~kevin/index.html

"There is a freedom lying beyond circumstance,
derived from the direct intuition that life can
be grounded upon its absorption in what is
changeless amid change"
   -- Alfred North Whitehead

"Kevin O'Gorman" <kogorman@pacbell.net> writes:
> I've got 33150 rows which the EXPLAIN said at top level would be
> 80-byte rows.  VACUUM indicates 488 pages.  Does this compute?

Keep in mind that EXPLAIN's estimates of average row width are almost,
but not quite, entirely bogus.  I'm actually quite surprised that your
arithmetic produced a number as close to 8K as it did.

We need to start keeping stats about the average width of variable-
length columns; until we do, EXPLAIN can't possibly produce row width
estimates that are at all accurate.

            regards, tom lane