Investigate the reason why there are so many join graphs

[snowgy]

Run the example in ver_test.py (add_dataset branch) and examine the join graphs.

[snowgy]

Previously, there is a bug in the join graph search implementation causing join graphs like [tbl1 join tbl2, tbl2 join tbl3, tbl3 join tbl1] (there is a circle in the graph). This bug is fixed in #45 , alleviating the problem of too many join graphs.