[ 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; > } -- This message is automatically generated by JIRA. - If you think it was sent incorrectly contact one of the administrators: http://issues.apache.org/jira/secure/Administrators.jspa - For more information on JIRA, see: http://www.atlassian.com/software/jira
