Re: encouraging index-only scans

Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
> A client is testing a migration from 9.1 to 9.2, and has found that a
> large number of queries run much faster if they use index-only scans.
> However, the only way he has found to get such a plan is by increasing
> the seq_page_cost to insanely high levels (3.5). Is there any approved
> way to encourage such scans that's a but less violent than this?

Is the pg_class.relallvisible estimate for the table realistic? They
might need a few more VACUUM and ANALYZE cycles to get it into the
neighborhood of reality, if not.

Keep in mind also that small values of random_page_cost necessarily
decrease the apparent advantage of index-only scans. If you think 3.5
is an "insanely high" setting, I wonder whether you haven't driven those
numbers too far in the other direction to compensate for something else.

regards, tom lane