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From Army <qoz...@sbcglobal.net>
Subject [OPTIMIZER] Proposal for pushing join predicates into Unions posted to DERBY-805.
Date Mon, 13 Feb 2006 08:52:41 GMT
I have attached a description of the changes I plan to submit for DERBY-805 to 
that Jira issue.  This is a rather complicated enhancement so the description of 
the changes is pretty long.  In short, though, I outline a 6-step approach to 
pushing join predicates down into Unions:

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.

2 - Implement the "pushOptPredicate()" and "optimizeIt()" methods for 
UnionNodes.  The former method should take predicates that are passed into the 
UnionNode from outer queries, scope them (per step 1) for the left and right 
children of the UnionNode, and store them locally.  The latter method should 
then pass the scoped predicates down to both children so that they can use the 
predicates in their own optimize()/optimizeIt() calls.

3 - Take scoped predicates (created in step 1) that are pushed into the children 
result sets of a UnionNode (per step 2) and allow the the children to push the 
scoped predicates even further down the tree, until we eventually get them to a 
base table.

4 - Make sure predicates that are pushed down into subqueries of a UnionNode are 
correctly "pulled" back up (if they are unscoped) or discarded (if they are 
scoped) for every permutation seen during optimization.

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.

6 - And finally, when optimization is complete, make sure all relevant 
predicates are pushed down the tree one last time and left there, in preparation 
for code generation.

See DERBY-805.html for all the gory details.

I have made the changes described in this document locally and they all seem to 
work, with a couple of exceptions as noted at the end of the document.  I plan 
to break the changes down into separate patches where it's possible to do so, 
and will be posting those patches in the coming days.  In the meantime, if 
anyone has time to review this document and provide feedback, direction, or 
suggestions, I would be grateful.  As I myself am still trying to learn all the 
subtleties of Derby optimization, the more feedback I get, the better...

Thanks,
Army

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