Re: space for optimalization: DISTINCT without index

On Mon, Dec 12, 2005 at 18:35:07 +0100, Pavel Stehule <pavel.stehule@hotmail.com> 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.