Mike Mascari <mascarm@mascari.com> writes:
> I *think* the result of assuming 0 rows in a newly created
> table, until the next vacuum, would yield a significant increase in
> mailing-list traffic complaints to the tune of:
> "Why isn't PostgreSQL using my index?"
> followed by the usual
> "Did you run VACUUM ANALYZE?"
> So an assumption of 1000 rows was made, with 10 rows matching your WHERE
> clause.
Yup, exactly. The initial default statistics are set (with malice
aforethought) to provoke an indexscan. After you VACUUM, the optimizer
knows how large the table really is (ie, tiny), and so it decides that
looking at the index is a waste of time, it might as well just scan the
table. Load up some more data, VACUUM again, and you'll probably see an
indexscan used.
> after a refresh database the explain yields:
> index scan using xXxX (cost=0.00..8.14 rows=10 width=147)
> after a vacuum + vacuum analyze the explain yields:
> seq scan on acc xXxX A(cost=0.00..1.23 rows=1 width=147)
BTW, comparing those two cost numbers is pretty pointless since they are
based on different information about the size of the table.
regards, tom lane