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From "A B (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 01:10:26 GMT
    [ http://issues.apache.org/jira/browse/DERBY-805?page=comments#action_12366731 ] 

A B commented on DERBY-805:
---------------------------

Based on the 6 steps that I've outlined in DERBY-805.html, I plan to post four separate patches
for this issue, in the following order:

-- Phase 1 -- The first patch will implement the changes as described in "Step 5" of the document,
namely:

	5 - Ensure that the best access path for a UnionNode that pushes
		predicates is correctly saved during optimization and correctly
		retrieved when it comes time to finalize the query's overall
		access path.

The phase 1 patch shouldn't have any functional effect on the codeline; it's just an incremental
step toward the complete fix.

-- Phase 2 -- The second patch will add code for "Step 1", which is:

	1 - Add the ability to take a predicate and scope it to a target
		result set so that it can be pushed to that result set. 

The phase 2 patch will add code to the codeline that won't actually get executed until phase
3.  Thus, like the phase 1 patch, the phase 2 patch should have no functional effects on the
codeline.

-- Phase 3 -- The third patch will be the one that does the core of the predicate pushing/pulling.
 This patch will cover Steps 2, 3, 4, and 6 of the steps outlined in the document.  Once this
patch is committed, the changes for phase 1 and phase 2 will take effect--so phase 3 is also
where I'll add tests to make sure predicates are being pushed correctly.  Note that after
phase 3, any join predicates which are targeted for UNIONS will _always_ be pushed down into
the union (assuming it's possible to do so).  This means that, as discussed in the document,
there will be cases where Derby originally chose to do a Hash join using the predicate but
will now (after phase 3) push the predicate and do a nested loop join.  This problem will
then be addressed in phase 4.

-- Phase 4 -- The phase 4 patch will address any unresolved issues from the first three phases,
which right now means that it will include the necessary changes to allow the optimizer to
make a cost-based decision about whether or not it should push the predicates, instead of
always pushing them.

The vast majority of the changes will go into the code as described in DERBY-805.html, with
potential variations due to review comments and/or improvements/alterations that I find to
be necessary as I work.  So anyone who might have the opportunity to review any of the phase
1-4 patches will hopefully find it useful to read the respective parts of the document...

> 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
>
> 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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