Github user DaveBirdsall commented on a diff in the pull request:
https://github.com/apache/incubator-trafodion/pull/174#discussion_r45124335
--- 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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