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From DaveBirdsall <>
Subject [GitHub] incubator-trafodion pull request: [TRAFODION-1585] Allow MDAM on i...
Date Tue, 17 Nov 2015 21:32:41 GMT
Github user DaveBirdsall commented on a diff in the pull request:
    --- Diff: core/sql/optimizer/OptPhysRelExpr.cpp ---
    @@ -3910,6 +3910,20 @@ InputPhysicalProperty* NestedJoin::generateIpp(
       InputPhysicalProperty* ipp = NULL;
       // ----------------------------------------------------------------
    +  // If there is a cardinality constraint on the outer child, we'll
    +  // pass that information along to the inner child.
    +  // ----------------------------------------------------------------
    +  Cardinality minRows;
    +  Cardinality maxRows;
    +  CardConstraint * outerCardinalityConstraint = NULL;
    +  if (getGroupAttr()->hasCardConstraint(minRows, maxRows))
    --- End diff --
    Yes, the intent of the change is to send any cardinality constraint on the result of the
left child down to the right child scan as a constraint on the number of probes. Before this
change, no such constraint was passed down; we relied on estimates only. The difference is
a matter of certainty. MDAM was considered dangerous to do on the inner child of a nested
join due to unreliability of estimates in some plans.

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