On Mon, Dec 12, 2005 at 18:35:07 +0100, Pavel Stehule <[EMAIL PROTECTED]> wrote: > Hello > > I did some test and I can see so DISTINCT works well on indexed columns, > but is slow on derived tables without indexes. If I use without distinct > group by I get much better times. > > SELECT DISTINCT a, b FROM tab > SELECT a,b FROM tab GROUP BY a, b. > > Can You Explain it.
DISTINCT will require a sort step to remove duplicates. GROUP BY can use either a sort or hash aggregate plan. If there are few distinct values, the hash aggregate plan can be much faster. ---------------------------(end of broadcast)--------------------------- TIP 6: explain analyze is your friend
