Consider the following nested SQL statement.
It seems that the cardinality of the SelectNode should be 1 instead of 0. Specifically, if we had executed
before issuing this SQL statement at the very beginning, the returned cardinality of this SelectNode would be 1 instead of 0. Not very sure if this is a bug. It looks like the cardinality of a SelectNode depends on whether there is stats information associated with its child node. The cardinality of a SelectNode would still be 0 even if its child node (ExchangeNode in this case) has a non-zero cardinality.