xiong duan created CALCITE-7053:
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             Summary: In HepPlanner, move down the collectGarbage to 
topological order for better optimizer performance
                 Key: CALCITE-7053
                 URL: https://issues.apache.org/jira/browse/CALCITE-7053
             Project: Calcite
          Issue Type: Bug
            Reporter: xiong duan


According to the java comment in HepPlanner#getGraphIterator:
{code:java}
    // Make sure there's no garbage, because topological sort
    // doesn't start from a specific root, and rules can't
    // deal with firing on garbage.
    // FIXME jvs 25-Sept-2006:  I had to move this earlier because
    // of FRG-215, which is still under investigation.  Once we
    // figure that one out, move down to location below for
    // better optimizer performance.
{code}
Based on the current implementation, I haven't found the reason why the 
depth-first order requires garbage collection every time. Since it always 
starts the search from a specific
Vertex, the issue mentioned the FRG-215 -
 I haven't found any description or information about this problem. Therefore, 
this PR attempts to delegate the collectGarbage operation to the topological 
order.



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