Re: optimizer question

Bruce Momjian wrote:
>
> > Hannu Krosing <hannu(at)tm(dot)ee> writes:
> > > Maybe rather
> >
> > > * Use indexes for min() and max() or convert to "SELECT col FROM tab
> > > ORDER BY col DESC USING max_index_op LIMIT 1" if there is an index
> > > on tab that uses btree(col max_index_op)
> >
> > > it seems that in most other cases the rewrite would be either a
> > > misoptimisation or plain wrong.
> >
> > We would clearly need to add information to the system catalogs to allow
> > the planner to determine whether a given aggregate matches up to a given
> > index opclass. This has been discussed before.
> >
> > A more interesting question is how to determine whether such a rewrite
> > would be a win. That is NOT a foregone conclusion. Consider
> >
> > SELECT max(col1) FROM tab WHERE col2 BETWEEN 12 AND 42;
> >
> > Depending on the selectivity of the WHERE condition, we might be far
> > better off to scan on a col2 index and use our traditional max()
> > code than to scan on a col1 index until we find a row passing the
> > WHERE condition. I'm not sure whether the planner currently has
> > statistics appropriate for such estimates or not ...
>
> Yes, agreed. This would be just for limited cases. Updated to:
>
> * Use indexes for min() and max() or convert to SELECT col FROM tab ORDER
> BY col DESC LIMIT 1 if appropriate index exists and WHERE clause acceptible
> ^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^
It would be probably a win if only exact match of

SELECT MAX(*) FROM TAB ;

would be rewritten if appropriate index exists.

The appropriateness should be explicitly declared in aggregate
definition.

-----------------
Hannu