Disallow joins whose lateral references need an unformable outer join If a LATERAL subquery references a PlaceHolderVar that must be evaluated at an outer join, the subquery's lateral_relids include that outer join's relid, since the lateral parameter is the outer join's output. join_is_legal() verified that the rels named by a proposed join's minimum parameterization could still be joined to from outside, but it did not consider such outer-join relids. As a result, when identity 3 permitted a commuted join order, we could approve a join that includes part of that outer join's required input, even though the outer join could then be completed only above the proposed join, leaving the lateral parameter forever unsatisfiable.
In assert-enabled builds this tripped the backstop Assert in try_nestloop_path(). In production builds the bogus join generated only paths whose parameterization can never be satisfied, so they could never appear in a complete plan. This not only wasted planning effort, but also could mislead the clauseless-join heuristics into thinking that legality of this join means that some other join rel need not be formed, and that could lead to failure to find any plan at all. Such joins used to be rejected by the have_dangerous_phv() check in join_is_legal(), which refused any join where a PlaceHolderVar's eval set overlapped the other side of the join without being contained in it. Commit a16ef313f removed that restriction, exposing this oversight. To fix, teach join_is_legal() to reject a proposed join if its minimum parameterization includes an outer-join relid whose outer join cannot be formed strictly outside the join, that is, when any rel needed to form that outer join must become part of the proposed join's own join tree. Back-patch to v18, where commit a16ef313f removed the have_dangerous_phv() restriction. Author: Richard Guo <[email protected]> Reviewed-by: Tom Lane <[email protected]> Reviewed-by: Tender Wang <[email protected]> Discussion: https://postgr.es/m/CAMbWs49ZveAYx9T5b0=gxn0pnvqq=mbn-z4klblae+kh06m...@mail.gmail.com Backpatch-through: 18 Branch ------ REL_18_STABLE Details ------- https://git.postgresql.org/pg/commitdiff/cbec572918fd1dd197e13e21952d2b5f0be2e942 Modified Files -------------- src/backend/optimizer/path/joinrels.c | 26 +++++++++++++++++++++++ src/test/regress/expected/join.out | 40 +++++++++++++++++++++++++++++++++++ src/test/regress/sql/join.sql | 20 ++++++++++++++++++ 3 files changed, 86 insertions(+)
