Avoid unnecessary wrapping for Vars and PHVs When pulling up a lateral subquery that is under an outer join, the current code always wraps a Var or PHV in the subquery's targetlist into a new PlaceHolderVar if it is a lateral reference to something outside the subquery. This is necessary when the Var/PHV references the non-nullable side of the outer join from the nullable side: we need to ensure that it is evaluated at the right place and hence is forced to null when the outer join should do so. However, if the referenced rel is under the same lowest nulling outer join, we can actually omit the wrapping. That's safe because if the subquery variable is forced to NULL by the outer join, the lateral reference variable will come out as NULL too. It could be beneficial to get rid of such PHVs because they imply lateral dependencies, which force us to resort to nestloop joins.
This patch leverages the newly introduced nullingrel_info to check if the nullingrels of the subquery RTE are a subset of those of the laterally referenced rel, in order to determine if the referenced rel is under the same lowest nulling outer join. No backpatch as this could result in plan changes. Author: Richard Guo Reviewed-by: James Coleman, Dmitry Dolgov, Andrei Lepikhov Discussion: https://postgr.es/m/CAMbWs48uk6C7Z9m_FNT8_21CMCk68hrgAsz=z6zpp1pnzmk...@mail.gmail.com Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/f64ec81a810ebd4649beb6c153844fa9ae1ecffe Modified Files -------------- src/backend/optimizer/prep/prepjointree.c | 39 ++++- src/test/regress/expected/subselect.out | 275 ++++++++++++++++++++++++++++++ src/test/regress/sql/subselect.sql | 70 ++++++++ 3 files changed, 375 insertions(+), 9 deletions(-)
