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From Apache Wiki <wikidi...@apache.org>
Subject [Db-derby Wiki] Trivial Update of "QueryPlanJoinOrder" by BryanPendleton
Date Sat, 11 Apr 2009 19:19:49 GMT
Dear Wiki user,

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The following page has been changed by BryanPendleton:

The comment on the change is:
Capture useful suggestion from Mamta

  This page was compiled with information posted to the derby-dev mailing list by Manjula,
Kathey, Army and Knut.
+ == Controlling Join Order in a test situation ==
+ Sometimes, it can be convenient to control the join order. For example, when debugging a
problem, it can be useful to force the optimizer to consider only a particular join order.
In [https://issues.apache.org/jira/browse/DERBY-3926 DERBY-3926] such a case arose, and Mamta
noted that it was useful to control the Optimizer's behavior using query overrides:
+ {{{
+ SELECT table1.id, m0.value, m1.value FROM --DERBY-PROPERTIES joinOrder=FIXED
+ table2 m1 -- DERBY-PROPERTIES index=key3
+ , table2 m0 -- DERBY-PROPERTIES index=key3
+ , table1
+ WHERE table1.id=m0.id AND m0.name='PageSequenceId' AND table1.id=m1.id AND m1.name='PostComponentId'
AND m1.value='21857' ORDER BY m0.value;
+ }}}
+ By controlling the join order, you may be able to get a simple repro case (ie you don't
have to go through countless iteration of optimizer for all different join orders and different
predicate pulling down in different join orders), which may make it easier to focus on the
problem join order.

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