Thread: encouraging index-only scans

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?

cheers

andrew

On 12/12/2012 04:32 PM, Tom Lane wrote:
> Andrew Dunstan <andrew@dunslane.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

On 12/12/2012 05:12 PM, Andrew Dunstan wrote:
>
> On 12/12/2012 04:32 PM, Tom Lane wrote:
>> Andrew Dunstan <andrew@dunslane.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.
>

Actually, the table had been analysed but not vacuumed, so this kinda
begs the question what will happen to this value on pg_upgrade? Will
people's queries suddenly get slower until autovacuum kicks in on the table?

cheers

andrew

Andrew Dunstan <andrew@dunslane.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