[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.

["A B (JIRA)" <de...@db.apache.org>]

     [ http://issues.apache.org/jira/browse/DERBY-805?page=all ]

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

    Attachment: d805_phase3_v1.patch
                d805_phase3_v1.stat

Attaching a "Phase 3" patch, d805_phase3_v1.patch, to implement the changes described in steps 2, 3, 4 and 6 of DERBY-805_v3.html.  In short, this patch does the following:

1. Implements the changes as described in the HTML document attached to this issue (steps 2, 3, 4 and 6).

2. Adds a check in PredicateList.java to skip scoped predicates when trying to find a predicate to use for a hash join.  This check is needed because if the predicate was scoped and pushed down from an outer query, then one of the operands points to the outer query and the other (scoped) operand points down into a subquery, which means we can't do the hash join.  Without this check the optimizer might choose to do a hash scan using the predicate even though it won't actually be able to do the corresponding hash join.  That means we would lose the opportunity to use the predicate for an index scan, and thus we could miss out on a much better (index-based) plan.

3. Makes a slight change to OptimizerImpl.addOrLoadBestPlanMappings to avoid doing extra work where possible.  More specifically, skips the logic to save/restore bestJoinOrder if there is only one Optimizable in the list, as there's only one possible join order in that case and so we don't need to keep track of which join order is best for each of the outer queries.

4. Adds a new test, lang/predicatePushdown.sql, to the harness as part of the derbylang suite.  This test runs a bunch of queries that relate to the pushing of predicates down into unions.  Most of these test cases failed (either with compilation/execution errors or incorrect results) at one point or another while I worked on this issue, so I've chosen to include them all as part of the new test.  The only way (that I know of) to tell if predicate pushdown is actually occuring is to print out the query plans, so the master file for this test is very large--almost 10,000 lines.  For that reason, combined with the fact that I think additional predicate pushdown tests will be needed as support for pushdown grows, I decided to create a new test instead of adding the test cases to an existing test.

Once the Phase 1, Phase 2, and Phase 3 patches have been applied, along with the patch for DERBY-1007, the basic predicate pushdown functionality will be in place.  That said, there are still two outstanding issues into which I'm looking:

A - As described in DERBY-805_v3.html, Phases 1 thru 3 will make it so that join predicates targeted for UNION nodes will always get pushed if it's possible to do so, which in turn means that the optimizer will no longer consider the cost of doing a hash join with the UNION (the hash join would require that the predicate _not_ be pushed into the UNION).  So I still need to investigate this to see how I can make the optimizer cost pushing predicates and using a nested loop join verses not pushing them and using a hash join.

B - The changes I've made for this issue--esp. for Phase 1--add a good amount of code to the optimization code path.  This means that it's now possible for the optimizer to timeout "sooner" than it would have prior to these changse.  By "sooner" I mean that the optimizer might not have a chance to look at as many join orders as it used to (because it takes longer for it to cost each join order, esp. with deeply nested subqueries).  This means that there are situations where the optimizer pre-DERBY-805 may have found one plan but post-DERBY-805 may never get to try that plan, and so it could end up choosing a worse plan than it used to. So I need to investigate the optimizer "timeout" mechanism to see how I can address this issue.

As originally stated when I broke this enhancement down into phases, the issues that remain after Phase 3 will be addressed in Phase 4 (and whatever follow-up phases are required thereafter). In the meantime, the surest way to get the full effect of predicate pushdown is to disable optimizer timeout, which can be done with the property derby.optimizer.noTimeout=true.  Note that I set this property for the new predicatePushdown test, to ensure that the optimizer will choose the same plans across all machines.

I ran derbyall on Red Hat Linux with IBM 1.4.2 and saw no new failures.  ** NOTE ** though that this patch, d805_phase3_v1.patch, has a dependency on d805_phase2_v1.patch and also on the patch for DERBY-1007, attached to that issue and called d1007_v1.patch.  So both of those patches must be applied before d805_phase3_v1.patch can be applied.

I know the patch looks long, but keep in mind that a good 9,000 lines of the diff is just for the new test.  I thought about posting the new test separately before the Phase 3 patch and then submitting an updated master file for Phase 3, but then the diff there would be even larger as most of the query plans would change.  So I've just included the new test as part of the Phase 3 patch.

So in short, the current patches for review-and-commit are as follow:

1. d805_phase1_v3.patch (already committed)
2. d805_phase2_v1.patch
3. d1007_v1.patch (for DERBY-1007; dependent on #1)
4. d805_phase3_v1.patch (dependent on all of the previous patches).

If anyone has the time to review/comment on any of these patches, I would most certainly appreciate it...

> 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: 10.2.0.0, 10.1.2.0
>  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_phase1_v3.patch, d805_phase1_v3.stat, d805_phase2_v1.patch, d805_phase2_v1.stat, d805_phase3_v1.patch, d805_phase3_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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