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find_var_for_subquery_tle() searches a subquery rel's targetlist for a Var referencing a given subquery output column, and asserted that any Var it finds there belongs to that rel. That does not hold for an appendrel child. set_append_rel_size() builds the child's targetlist by translating the parent's, and as noted there the result can contain arbitrary expressions. In particular, when a member of a LATERAL UNION ALL subquery is pulled up and its output is a lateral reference, the translated expression is a Var of the laterally referenced rel. If the child is itself a subquery with sorted paths, convert_subquery_pathkeys() would trip the Assert. In non-assert builds, the function went on to compare such a Var's varattno with the subquery column's resno, and so could match a Var of an unrelated rel. To fix, ignore Vars that do not belong to the subquery rel, the same way set_rel_width() already does. Also correct the adjacent comment, which claimed that the only non-Var entries are placeholders. Back-patch to all supported branches. Author: Richard Guo <guofenglinux@gmail.com> Reviewed-by: shihao zhong <zhong950419@gmail.com> Discussion: https://postgr.es/m/CAMbWs48fFiPi7Q5yJCoM_nAs47zd5XgvPHd9nZyM_ewq+k_jpQ@mail.gmail.com Backpatch-through: 14