But just because you have 100 values in no way indicates how many rows will be returned. And I personally know of no optimizers that make such an assumption; its a totally worthless assumption.
Nor is this "lack of de-dupping" any real issue that that I can think of. On Thu 17 May 2012 07:16:06 AM CDT, Guillaume Smet wrote: > Hi Steve, > > On the list again, my bad... > > On Thu, May 17, 2012 at 1:07 PM, Steve Ebersole<st...@hibernate.org> wrote: >> I am not following what you are saying about "reusing keys" and optimizers. >> Nor how it might lead to bad execution plans. Can you elaborate on your >> thoughts here? > > If you have a lot of (OR primaryKey=value) clauses with the same > value, if the optimizer doesn't deduplicate the clauses, the > statistics used by the planner to build the query plan might be wrong > and lead to bad query plans. > > At least some of the optimizers out there don't remove identical > values as it's a lot of cycles usually wasted for nothing as nobody > writes query like that. > > Simplified example: if you have 100 (OR primaryKey=1) clauses, the > planner might consider that you'll have 100 values returned which is > plain wrong in this case. It's not important if the query is simple > and you don't have a lot of joins but if you're in the case of a > complex object hierarchy with a lot of left joins, it might lead to > really bad execution plans. > -- st...@hibernate.org http://hibernate.org _______________________________________________ hibernate-dev mailing list hibernate-dev@lists.jboss.org https://lists.jboss.org/mailman/listinfo/hibernate-dev
