[ http://issues.apache.org/jira/browse/DERBY-805?page=comments#action_12366756 ]
Jeff Lichtman commented on DERBY-805: ------------------------------------- > But I wonder if that couldn't be done later as follow-up work? Would you say > that the changes I've proposed--namely, see if we can push the predicates to > both sides and skip the push if not--are incorrect, or just that we could > build on them to make them better? Your changes are not incorrect. I just wonder whether they wouldn't be both simpler and more complete if you didn't have to worry about whether predicates were pushed to both sides. I have the feeling you're anxious to finish this phase of the project. It's OK with me if you check in this change with the current algorithm. If you do this, please add comments to the code saying what could be changed to make it possible to apply a pushed predicate to one side only. > Push join predicates into union and other set operations. DERBY-649 > implemented scalar (single table) predicate pushdown. Adding join predicate > push down could improve performance significantly. > -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- > > Key: DERBY-805 > URL: http://issues.apache.org/jira/browse/DERBY-805 > Project: Derby > Type: Sub-task > Components: SQL > Versions: 10.1.2.0, 10.2.0.0 > Environment: generic > Reporter: Satheesh Bandaram > Assignee: A B > Fix For: 10.2.0.0 > Attachments: DERBY-805.html, d805_phase1_v1.patch, d805_phase1_v1.stat > > Fix for DERBY-649 implemented scalar (single table) predicate push down into > UNIONs. While this improves performance for one set of queries, ability to > push join-predicates further improves Derby performance by enabling use of > indices where possible. > For example, > create view V1 as select i, j from T1 union all select i,j from T2; > create view V2 as select a,b from T3 union all select a,b from T4; > insert into T1 values (1,1), (2,2), (3,3), (4,4), (5,5); > For a query like > select * from V1, V2 where V1.j = V2.b and V1.i =1; > If the join order choosen is V1,V2, V1 can use index on V1.i (if present) > following fix for DERBY-649. But if there is a index on V2.b also, Derby > currently can't use that index. By pushing join predicate, Derby would be > able to use the index and improve performance. Some of the queries I have > seen (not the one shown here...) could improve from 70-120 seconds to about > one second. > Note there is a good comment by Jeff Lichtman about join-predicate push down. > I am copying parts of it here for completeness of this report: (Modified) > If predicate push down is done during optimization, it would be possible to > push joins into the union as long as it's in the right place in the join > order. > For example: > create view v as select * from t1 union all select * from t2; > select * from v, t3 where v.c1 = t3.c2; > In this select, if t3 is the outer table then the qualification could be > pushed into the union and optimized there, but if t3 is the inner table the > qualification can't be pushed into the union. > If the pushing is done at preprocess time (i.e. before optimization) it is > impossible to know whether a join qualification like this can be safely > pushed. > There's a comment in UnionNode.optimizeIt() saying: > /* RESOLVE - don't try to push predicated through for now */ > This is where I'd expect to see something for pushing predicates into the > union during optimization. > BTW, the business of pushing and pulling predicates during optimization can > be hard to understand and debug, so maybe it's best to only handle the simple > cases and do it during preprocessing. -- This message is automatically generated by JIRA. - If you think it was sent incorrectly contact one of the administrators: http://issues.apache.org/jira/secure/Administrators.jspa - For more information on JIRA, see: http://www.atlassian.com/software/jira
