A Dimecres 21 Maig 2008, Richard Huxton va escriure:
Albert Cervera Areny wrote:
I've got a query similar to this:
select * from t1, t2 where t1.id > 158507 and t1.id = t2.id;
That took > 84 minutes (the query was a bit longer but this is the part
that made the difference) after a little change the query took ~1 second:
select * from t1, t2 where t1.id > 158507 and t2.id > 158507 and t1.id =
t2.id;
Try posting EXPLAIN ANALYSE SELECT ... for both of those queries and
we'll see why it's better at the second one.
Even if the estimates were off (they look a bit off for the first
table), the above two queries are logically identical, and I would
expect the planner to make the same decision for both.
I am curious - what is the result of:
select * from t1, t2 where t2.id > 158507 and t1.id = t2.id;
Is it the same speed as the first or second, or is a third speed entirely?
If t1.id = t2.id, I would expect the planner to substitute them freely
in terms of identities?
Cheers,
mark
--
Mark Mielke <mark@xxxxxxxxx>
