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=z6zpP1PNZMkeoQ@mail.gmail.com
Branch
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master
Details
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https://git.postgresql.org/pg/commitdiff/f64ec81a810ebd4649beb6c153844fa9ae1ecffe
Modified Files
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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(-)
