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From "Satheesh Bandaram (JIRA)" <derby-...@db.apache.org>
Subject [jira] Commented: (DERBY-781) Materialize union subqueries in select list where possible to avoid creating invariant resultsets many times.
Date Fri, 21 Jul 2006 19:19:14 GMT
    [ http://issues.apache.org/jira/browse/DERBY-781?page=comments#action_12422753 ] 
            
Satheesh Bandaram commented on DERBY-781:
-----------------------------------------

I think it would be good to modify this improvement description, as it will likely be picked
up by release notes and/or other documentation. The fix is more generic than 'UNION' subqueries
as the original description says.

Also the example in the description doesn't apply anymore, I think.  When the entry was made,
join-predicate push down work wasn't completed, so the example in the description would have
shown the problem, I think. But now, (post join-predicate pushdown work) the example may not
apply.



> Materialize union subqueries in select list where possible to avoid creating invariant
resultsets many times.
> -------------------------------------------------------------------------------------------------------------
>
>                 Key: DERBY-781
>                 URL: http://issues.apache.org/jira/browse/DERBY-781
>             Project: Derby
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 10.1.1.0, 10.2.0.0
>         Environment: generic
>            Reporter: Satheesh Bandaram
>         Assigned To: A B
>         Attachments: d781_v1.patch, d781_v1.stat, d781_v2.patch, DERBY-781_v1.html
>
>
> Derby's handling of union subqueries in from list can be improved by materializing invariant
resultsets once, rather than creating them many times.
> For example:
> create view V1 as select i, j from T1 union select i,j from T2;
> create view V2 as select a,b from T3 union 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 in (1,2,3,4,5), it is
possible the resultset for V2 is created 5 times. (assuming V2 is choosen as the the inner
table) This can be very costly if the underlying selects can take long time and also may perform
union many times.
> Enhance materialization logic in setOperatorNode.java. It currently returns FALSE always.
> public boolean performMaterialization(JBitSet outerTables)
> 		throws StandardException
> {
> 	// RESOLVE - just say no to materialization right now - should be a cost based decision
> 	return false;
> 	/* Actual materialization, if appropriate, will be placed by our parent PRN.
> 	 * This is because PRN might have a join condition to apply.  (Materialization
> 	 * can only occur before that.
> 	 */
> 	//return true;
> } 

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