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From "A B (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 20:23:16 GMT
    [ http://issues.apache.org/jira/browse/DERBY-781?page=comments#action_12422767 ] 
            
A B commented on DERBY-781:
---------------------------

>  I think it would be good to modify this improvement description

Seems like this could be a simple as removing the word "union" from the description--does
that sound reasonable to you?  Or do you want an entirely new description?  Something like
"Subquery materialization via hash join" or "Support subquery materialization by allowing
the optimizer to cost and generate hash joins with subqueries".

Any preference?

> 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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