Github user DaveBirdsall commented on a diff in the pull request:
https://github.com/apache/incubator-trafodion/pull/174#discussion_r45080794
--- Diff: core/sql/optimizer/ScanOptimizer.cpp ---
@@ -3364,7 +3364,25 @@ ScanOptimizer::isMdamEnabled() const
// If # of probes for NJ is <= cqd(MDAM_UNDER_NJ_PROBES_THRESHOLD),
// we will use MDAM regardless of the setting of CQD MDAM_SCAN_METHOD.
- // When the CQD is 0, MDAM under NJ is not considered.
+ // When the CQD is 0, MDAM under NJ is not considered unless we
+ // have a cardinality constraint on the number of probes that
+ // guarantees at most one probe.
+
+ const InputPhysicalProperty* ippForMe =
+ getContext().getInputPhysicalProperty();
+ if (ippForMe)
+ {
+ const CardConstraint * outerCardConstraint =
+ ippForMe->getOuterCardConstraint();
+ if (outerCardConstraint)
+ {
+ Cardinality upperBound = outerCardConstraint->getUpperBound();
+ if (upperBound <= (Cardinality)1)
--- End diff --
There is already a CQD just a few lines down in the code that will allow
this for estimates. I wanted to avoid introducing yet another CQD. That was the
point of the code a few lines down that used the cardinality constraint to
sharpen the row estimate.
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