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From "A B (JIRA)" <derby-...@db.apache.org>
Subject [jira] Updated: (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 Wed, 22 Feb 2006 21:05:38 GMT
     [ http://issues.apache.org/jira/browse/DERBY-805?page=all ]

A B updated DERBY-805:
----------------------

    Attachment: d805_phase2_v1.patch
                d805_phase2_v1.stat
                DERBY-805_v3.html

I've finalized changes for d805_phase1_v3.patch but still need to run derbyall tonight to
make sure there are no new diffs (I meant to do it last night but ended up tracing through
code well into the night instead).

In the meantime, I'm posting the first version of the Phase 2 patch here: d805_phase2_v1.patch.
 This patch handles the "scoping" logic for predicates as described in "Step 1" of DERBY-805_v3.html,
which I'm also posting here as there have been some updates since the first version.

The Phase 2 patch (d805_phase2_v1.patch) is independent of the Phase 1 patch and therefore
can be committed before or after Phase 1--the order in which the two are committed doesn't
actually matter so long as both are committed before Phase 3.

The Phase 2 patch does the following:

  1. Adds the necessary logic/methods to scope a predicate
  2. Adds some utility methods and one new class (BaseTableNumbersVisitor)
	used for scoping predicates
  3. Adds some other simple utility methods in preparation for predicate
	pushdown with Phase 3.

None of the new methods will ever get called until Phase 3 is committed, so d805_phase2_v1.patch
should have no affect on the codeline--it's just part of incremental development.

Thanks to anyone who has a chance to review and/or commit...

> 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, DERBY-805_v2.html, DERBY-805_v3.html, d805_phase1_v1.patch,
d805_phase1_v1.stat, d805_phase1_v2.patch, d805_phase1_v2.stat, d805_phase2_v1.patch, d805_phase2_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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