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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 Sun, 19 Feb 2006 03:46:47 GMT
     [ http://issues.apache.org/jira/browse/DERBY-805?page=all ]

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

    Attachment: d805_phase1_v2.patch
                d805_phase1_v2.stat
                DERBY-805_v2.html

Attaching a "version 2" of the Phase 1 patch, which does things in a more general and more
robust way, based on discussed here:

http://article.gmane.org/gmane.comp.apache.db.derby.devel/14934

The new patch is "d805_phase1_v2.patch".  I'm also attaching DERBY-805_v2.html, which is a
new version of the document that has been updated with details to explain the new approach
for Phase 1 patch--see "Step 5" of section VI.  This new description matches what was said
on derby-dev, with one minor change: the call to addOrLoadBestPlan() when placing an Optimizable
was removed, as it turns out that it wasn't needed.  The new description also has a walk-through
of what the new code does for a given example.

Note that even though d805_phase1_v2.patch is a different approach, it is still the case none
of the existing Derby functionality should be affected by this change.  The new code is exercised,
but until we actually start pushing predicates, there should be no behavioral differences.

I ran derbylang with this patch and IBM 1.4.2 with no failures.  I haven't had time to run
derbyall yet, but hope to do so tonight (my poor machine is working around the clock lately,
so night-time is the only time I can run the full suite).

Thanks in advance for review comments.

> 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, d805_phase1_v1.patch, d805_phase1_v1.stat,
d805_phase1_v2.patch, d805_phase1_v2.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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