Re: Order by optimisations?

Hannu Krosing wrote:
> On K, 2005-07-13 at 16:08 +0800, Christopher Kings-Lynne wrote:
> 
>>Hi,
>>
>>Does PostgreSQL do the following optimisation:
>>
>>SELECT * FROM diary WHERE date = '2005-05-01' ORDER BY date;
>>
>>or in fact even better (for my situation)
>>
>>SELECT * FROM diary WHERE date BETWEEN '2005-05-01' AND '2005-05-01' 
>>ORDER BY date;
>>
>>Does it know that the input to the sort routine is already sorted and 
>>hence is a no-op?
> 
> 
> Yes
> 
> try EXPLAIN ;)

Doesn't seem like it does:

usatest=# explain select * from users_myfoods_map where 
date='2004-11-21' order by date;                                QUERY PLAN
--------------------------------------------------------------------------- Sort  (cost=17.17..17.48 rows=123 width=22)
 Sort Key: date   ->  Seq Scan on users_myfoods_map  (cost=0.00..12.90 rows=123 width=22)         Filter: (date =
'2004-11-21'::date)
(4 rows)

The sort cost is non-zero.  Or am I not looking at the right thing...

Chris