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From Jeffrey Lichtman <swa...@rcn.com>
Subject Re: [OPTIMIZER] Proposal for pushing join predicates into Unions posted to DERBY-805.
Date Mon, 13 Feb 2006 09:11:50 GMT

>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.

Predicates may have to be copied when pushed into the children of a 
UnionNode. When the predicates are pulled back up out of the 
UnionNode, all but one copy of each will have to be discarded. Please 
watch out for whether this causes performance problems due to object 
creation and garbage collection. If such problems exist, it may be 
possible to cache copies of predicates for re-use.

                        -        Jeff Lichtman
                                 Check out Swazoo Koolak's Web Jukebox at

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