Re: Optimizer internals

[Bruno Wolff III <bruno(at)wolff(dot)to>]

On Thu, Jun 15, 2006 at 15:38:32 -0400,
  John Vincent <lusis(dot)org(at)gmail(dot)com> wrote:
> Any suggestions? FYI the original question wasn't meant as a poke at
> >comparing PG to MySQL to DB2. I'm not making an yvalue judgements either
> >way. I'm just trying to understand how we can use it the best way possible.
> >
> 
> Actually we just thought about something. With PG, we can create an index
> that is a SUM of the column where indexing, no? We're going to test this in
> a few hours. Would that be able to be satisfied by an index scan?

No, that won't work. While you can make indexes on functions of a row, you
can't make indexes on aggregate functions.

You might find making a materialized view of the information you want can
help with performance. The issues with "sum" are pretty much the same ones
as with "count". You can find a couple different ways of doing materialized
views for "count" in the archives. There is a simple way of doing it that
doesn't work well with lots of concurrent updates and a more complicated
method that does work well with lots of concurrent updates.