Re: Querying distinct values from a large table

On Tue, Jan 30, 2007 at 14:33:34 +0600,
Igor Lobanov <ilobanov(at)swsoft(dot)com> wrote:
> Greetings!
>
> I have rather large table with about 5 millions of rows and a dozen of
> columns. Let's suppose that columns are named 'a', 'b', 'c' etc. I need
> to query distinct pairs of ('a';'b') from this table.
>
> Is there any way to somehow improve the performance of this operation?
> Table can not be changed.

DISTINCT currently can't use a hash aggregate plan and will use a sort.
If there aren't many distinct values, the hash aggregate plan will run much
faster. To get around this limitation, rewrite the query as a group by.
Something like:
SELECT a, b FROM table GROUP BY a, b;