On Thu, 2006-05-25 at 16:52 -0400, Tom Lane wrote:
> "Merlin Moncure" <mmoncure@gmail.com> writes:
> > been doing a lot of pgsql/mysql performance testing lately, and there
> > is one query that mysql does much better than pgsql...and I see it a
> > lot in normal development:
>
> > select a,b,max(c) from t group by a,b;
>
> > t has an index on a,b,c.
>
> The index won't help, as per this comment from planagg.c:
>
> * We don't handle GROUP BY, because our current implementations of
> * grouping require looking at all the rows anyway, and so there's not
> * much point in optimizing MIN/MAX.
>
> Given the numbers you mention (300k rows in 2000 groups) I'm not
> convinced that an index-based implementation would help much; we'd
> still need to fetch at least one record out of every 150, which is
> going to cost near as much as seqscanning all of them.
Well, if the MySQL server has enough RAM that the index is cached (or
index + relevant chunks of data file if using InnoDB?) then that would
explain how MySQL can use an index to get fast results.
-- Mark Lewis
