From: Richard Guo <rguo@postgresql.org>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Collect quals for outer-join reduction on demand
Date: Fri, 21 Aug 2026 06:52:25 +0000
Message-ID: <E1wxJ72-00000001Tc4-325W@gemulon.postgresql.org> (raw)
Collect quals for outer-join reduction on demand
To prove that a forced-null Var is non-nullable in every matching row,
reduce_outer_joins() uses the strict quals that hold for every row a
subtree emits. The first pass currently gathers those quals into each
subtree's state, so that the second pass need not re-walk the jointree
at decision time.
That duplicates find_subquery_safe_quals(), which already performs
this traversal for query_outputs_are_not_nullable(), and it charges
every query containing an outer join for a list build at every
FromExpr and every inner or semi join, whether or not any upper qual
turns out to force a Var null. Since each level copies what its
children collected, the space required grows quadratically with the
depth of nested inner joins. What it buys is small even then: the
walk it avoids is cheap enough to be called on demand.
So rename find_subquery_safe_quals() to find_safe_quals(), export it,
and drop safe_quals from the first pass's state in favor of the
subtree's jointree node. The second pass calls find_safe_quals() at
the point where it needs the proof.
Also stop treating a semijoin there like a left join: semijoins are
reachable from the new caller, and a semijoin emits only matched
left-hand rows, so its ON quals do hold for every output row.
Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAMbWs4-6FazY7+vCOvR0-vqNuaXOZic8O4D4ZrwMm2woi+Q3uA@mail.gmail.com
Branch
------
master
Details
-------
https://git.postgresql.org/pg/commitdiff/31c002aa89787e4bbe237e36f51dead96b674998
Modified Files
--------------
src/backend/optimizer/prep/prepjointree.c | 61 ++++++-------------------------
src/backend/optimizer/util/clauses.c | 54 ++++++++++++++++-----------
src/include/optimizer/clauses.h | 1 +
3 files changed, 45 insertions(+), 71 deletions(-)
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To: pgsql-committers@postgresql.org
Cc: rguo@postgresql.org, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Collect quals for outer-join reduction on demand
In-Reply-To: <E1wxJ72-00000001Tc4-325W@gemulon.postgresql.org>
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