Github user jianqiao commented on a diff in the pull request:

    https://github.com/apache/incubator-quickstep/pull/171#discussion_r98545487
  
    --- Diff: query_optimizer/PhysicalGenerator.cpp ---
    @@ -109,6 +114,13 @@ P::PhysicalPtr PhysicalGenerator::optimizePlan() {
       } else {
         rules.emplace_back(new SwapProbeBuild());
       }
    +  if (FLAGS_reorder_columns) {
    +    // NOTE(jianqiao): This optimization relies on the fact that the 
intermediate
    +    // relations all have SPLIT_ROW_STORE layouts. If this fact gets 
changed, the
    +    // optimization algorithm may need to be updated and the performance 
impact
    +    // should be re-evaluated.
    --- End diff --
    
    Currently this cannot be checked because the temporary relations' layouts 
are generated in `ExecutionGenerator` when they get created.
    
    In the future we will reason about the layouts during query optimization 
and add corresponding data structures.


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