Materialize union subqueries in select list where possible to avoid creating
invariant resultsets many times.
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Key: DERBY-781
URL: http://issues.apache.org/jira/browse/DERBY-781
Project: Derby
Type: Improvement
Components: SQL
Versions: 10.1.1.0, 10.2.0.0
Environment: generic
Reporter: Satheesh Bandaram
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 all select i,j from T2;
create view V2 as select a,b from T3 union all 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 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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