A minor variation on check-in [1685610ef8e0dc] which (hopefully) makes the

A minor variation on check-in [1685610ef8e0dc] which (hopefully) makes the
logic a little easier to follow.  Also disallows the use of the double-quoted
string hack in the query expression used for VACUUM INTO, which is not
strictly required, but moves us toward the goal of disallowing the double-quoted
string hack everywhere.

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