Re: encouraging index-only scans

On 12/12/2012 04:32 PM, Tom Lane wrote:
> 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.

That was the problem - I didn't know this hadn't been done.

>
> 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.

Right.

Thanks for the help.

cheers

andrew