JOIN if that subquery reads data from a virtual table. Because it prevents the
planner from creating an automatic index on the results of the sub-query, the
flattening optimization sometimes slows things down in this case.
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LEFT JOIN and there is a MATCH constraint in the ON clause, or when the vtab
is in a sub-query that is the RHS of a LEFT JOIN and there is a MATCH
constraint in the WHERE clause of the sub-query.
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