Github user jianqiao commented on the issue:
https://github.com/apache/incubator-quickstep/pull/180
Estimated cardinality being zero means that we expect the relation to be
empty or have a small number of tuples. In either case, the created filter is
effective and can be used to pre-filter other relations. Otherwise if we don't
create such a filter, then there would be no optimization at all.
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