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From "A B (JIRA)" <derby-...@db.apache.org>
Subject [jira] Created: (DERBY-754) Push ON clause predicates down when optimizing SELECT queries.
Date Mon, 12 Dec 2005 16:51:45 GMT
Push ON clause predicates down when optimizing SELECT queries.
--------------------------------------------------------------

         Key: DERBY-754
         URL: http://issues.apache.org/jira/browse/DERBY-754
     Project: Derby
        Type: Improvement
  Components: Performance  
    Versions: 10.2.0.0, 10.1.2.0, 10.1.2.1    
    Reporter: A B
 Assigned to: A B 


In cases where a SELECT subquery occurs as one of the operands to a Join, it is sometimes
beneficial to push join predicates down to the subquery, which allows the optimizer to find
a better access path for the subquery and thus can improve query performance.

For example, take the following query:

SELECT  t1.a, t1.b, temp.c
FROM t1
  LEFT OUTER JOIN (
    SELECT c,d
    FROM t2
  ) as temp
  ON
    t1.a = temp.d
    and temp.d = 8
;

Currently, when optimizing the inner SELECT query, Derby will only pass the inner SELECT's
WHERE predicates to the optimizer--the outer ON predicates are ignored.  Thus, in this case,
the optimizer will have no predicates to work with and so will do a table scan on t2.

If, however, Derby were to push the "temp.d = 8" predicate down into the inner SELECT query,
the optimizer could use that predicate to make a smarter decision.  For example, if a primary
key existed on column "d" in T2, the optimizer could use that and then choose to do an index/hash
scan when reading t2 (instead of a table scan).

Not only does can this kind of "pushing" lead to faster reading of tables, but in some cases
where the predicate being pushed references two tables, it can also influence the optimizer's
choice of join strategy, which can in turn lead to improved performance.

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