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
---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
