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An expression such as WHERE (... x IN (SELECT ...) ...) IN (SELECT ...) could produce an invalid plan that results in a crash at execution time, if the planner attempts to flatten the outer IN into a semi-join. This happens because convert_testexpr() was not expecting any nested SubLinks and would wrongly replace any PARAM_SUBLINK Params belonging to the inner SubLink. (I think the comment denying that this case could happen was wrong when written; it's certainly been wrong for quite a long time, since very early versions of the semijoin flattening logic.) Per report from Teodor Sigaev. Back-patch to all supported branches.