Re: Schema design question as it pertains to performance

Thanks, Kevin.
That was my intent - if no column of an index changes in an update then no changes are done on the index.
That helps quite a bit - I will try moving the columns which change continuously to their own index - hopefully this will tame the overwhelming IO, since the index will now have a single column. I am seeing too many semwaits or ufs states when running top, which means we are not quite keeping up.
A lot of the load has been offloaded to the read only hot standby - thanks for the pointers on the delays. I only had one query cancelled today (as opposed to many previously), so it is just a matter of fine tuning right now.

-----Original Message-----
From: Kevin Grittner [mailto:kgrittn(at)mail(dot)com]
Sent: Tuesday, January 22, 2013 2:56 PM
To: Benjamin Krajmalnik; pgsql-admin(at)postgresql(dot)org
Subject: Re: [ADMIN] Schema design question as it pertains to performance

Benjamin Krajmalnik wrote:

> From a performance standpoint, is there a big hit on select
> performance if a query ends up utilizing more than one index, taking
> into account that an index has been used already to reduce the data
> set of potential records, and the secondary index would mostly be used
> in the ordering of the result set (such as a last updated time)?

That depends on so many variables it is hard to give a simple answer.

> I also assume that if no data has changed in an index, nothing is done
> when the record is updated as pertains to the particular index - am I
> correct in this assumption?

No. If the update doesn't affect *any* indexed column, and there is room in the page, it will do a HOT update and can skip all index updates. If any indexed column is updated, it must expire the old tuple and create a new tuple to represent the updated version of the row, and this new tuple is not likely to land in the same page as the old tuple; so it needs new entries in all the indexes. The old index entries must remain until they can no longer be the visible version of the row for any database transaction, so both versions of the row will be on the index for a while.

-Kevin