Major Description
1. Semi/anti joins: including "IN / NOT IN" tests operating on non-correlated sub-queries (PHOENIX-167).
We can evaluate the sub-queries as sets and execute the outer query the same way as we now do IN clause.
2. Other non-correlated sub-queries: having a comparison operator with no modifier or a comparison operator modified by ANY, SOME or ALL (PHOENIX-1168).
a) The sub-query can/must be evaluated as a single value. (Like "= / !=" with no modifiers; Or in cases like "> / < / >= / <=" "ANY/SOME/ALL", either max() or min() can be applied to the sub-query).
b) In cases like "= / !=" "ANY / SOME / ALL", they are equivalent to category 2 queries.
3. General correlated sub-queries (PHOENIX-945):
Need more sophisticated rewriting techniques to convert them into joins.
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- if none of the dependence of inner query on the outer query is equi condition, we are currently unable to handle them.
4. Nested sub-queries: having sub-queries in the FROM clause.
We are now able to handle most of them. Exceptions are PHOENIX-943, PHOENIX-944.
