Re: cost of CREATE VIEW ... AS SELECT DISTINCT

Alvaro Herrera wrote:
> On Tue, Mar 29, 2005 at 02:21:15PM +0100, T E Schmitz wrote:
>
>>Bruno Wolff III wrote:
>>
>>>On Tue, Mar 29, 2005 at 11:07:20 +0100,
>>> T E Schmitz <mailreg(at)numerixtechnology(dot)de> wrote:
>>>
>>>
>>>>Would the "SELECT DISTINCT origin" always cause a sequential table
>>>>scan regardless whether there is an index on the origin column or
>>>>not?
>>>
>>>It's worse than that, SELECT DISTINCT cannot use a hash aggregate
>>>plan and will need to do a sort to eliminate duplicates. Unless the
>>>view is used in a way that restricts the candidate rows, this
>>>probably isn't going to be very fast. You might be better off
>>>changing the view to use GROUP BY instead of DISTINCT.
>>
>>As far as I can see (via EXPLAIN), both DISTINCT and GROUP BY will
>>lead to a sequentail scan. Is that correct?
>
> That (GROUP BY using a seqscan) may be caused by the small size of the
> table. Try populating it some more.
>

Shall do.
But am I correct in assuming that I should place an index on the group
by (TRANSAKTION.ORIGIN) column?

--

Regards/Gruß,

Tarlika Elisabeth Schmitz