Re: optimizer question

> 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
^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^

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