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From jianqiao <...@git.apache.org>
Subject [GitHub] incubator-quickstep pull request #332: Small adjustments in star schema cost...
Date Fri, 23 Feb 2018 21:13:34 GMT
Github user jianqiao commented on a diff in the pull request:

    https://github.com/apache/incubator-quickstep/pull/332#discussion_r170369358
  
    --- Diff: query_optimizer/cost_model/StarSchemaSimpleCostModel.cpp ---
    @@ -493,7 +493,7 @@ std::size_t StarSchemaSimpleCostModel::getNumDistinctValues(
           return stat.getNumDistinctValues(rel_attr_id);
         }
       }
    -  return estimateCardinalityForTableReference(table_reference);
    +  return estimateCardinalityForTableReference(table_reference) * 0.1;
    --- End diff --
    
    This estimation ratio can be any decimal number that is not close to `1` -- in that case
the optimizer would choose bad plans in some situations as the column appears to have "unique"
values.
    
    `0.1` tends to be a reasonable choice -- we may also have `0.05`, `0.2`, etc., which can
be adjusted later when there are actual demands.



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