1cb108efb0
If a sub-select's output column is a simple Var, recursively look for statistics applying to that Var, and use them if available. The need for this was foreseen ages ago, but we didn't have enough infrastructure to do it with reasonable speed until just now. We punt and stick with default estimates if the subquery uses set operations, GROUP BY, or DISTINCT, since those operations would change the underlying column statistics (particularly, the relative frequencies of different values) beyond recognition. This means that the types of sub-selects for which this improvement applies are fairly limited, since most subqueries satisfying those restrictions would have gotten flattened into the parent query anyway. But it does help for some cases, such as subqueries with ORDER BY or LIMIT.
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