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http://issues.apache.org/jira/browse/DERBY-805?page=comments#action_12366731 ]
A B commented on DERBY-805:
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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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