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From "Jeff Lichtman (JIRA)" <derby-...@db.apache.org>
Subject [jira] Commented: (DERBY-805) 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.
Date Fri, 17 Feb 2006 06:59:28 GMT
    [ 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.

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