Github user jihoonson commented on the pull request:
https://github.com/apache/tajo/pull/593#issuecomment-121170531
@hyunsik thanks for your review. I've reflected all your comments and added
more comments to help your understanding.
Regarding on the question about JoinGraph::addJoin, it seems that there are
some misunderstandings. The JoinGraph::addJoin method just checks whether there
is a cycle rather than physically cutting it. This is to maintain the ```root
vertexes``` which the graph traverse is started from. In this patch, the root
vertexes are identified if they don't have any incoming edges. However, in the
presence of a cycle, every vertex has at least one incoming edge, so cannot be
the root vertex. The if clause related to the cycle at JoinGraph.java:43 is to
prevent such case.
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