trafodion-codereview mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From DaveBirdsall <...@git.apache.org>
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:

    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.


---
If your project is set up for it, you can reply to this email and have your
reply appear on GitHub as well. If your project does not have this feature
enabled and wishes so, or if the feature is enabled but not working, please
contact infrastructure at infrastructure@apache.org or file a JIRA ticket
with INFRA.
---

Mime
View raw message