Re: encouraging index-only scans

On 12/12/2012 05:12 PM, Andrew Dunstan wrote:
>
> 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.
>

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