Optimize joins when the inner relation can be proven unique.
If there can certainly be no more than one matching inner row for a given
outer row, then the executor can move on to the next outer row as soon as
it's found one match; there's no need to continue scanning the inner
relation for this outer row. This saves useless scanning in nestloop
and hash joins. In merge joins, it offers the opportunity to skip
mark/restore processing, because we know we have not advanced past the
first possible match for the next outer row.
Of course, the devil is in the details: the proof of uniqueness must
depend only on joinquals (not otherquals), and if we want to skip
mergejoin mark/restore then it must depend only on merge clauses.
To avoid adding more planning overhead than absolutely necessary,
the present patch errs in the conservative direction: there are cases
where inner_unique or skip_mark_restore processing could be used, but
it will not do so because it's not sure that the uniqueness proof
depended only on "safe" clauses. This could be improved later.
David Rowley, reviewed and rather heavily editorialized on by me
Discussion: https://postgr.es/m/CAApHDvqF6Sw-TK98bW48TdtFJ+3a7D2mFyZ7++=D-RyPsL76gw@mail.gmail.com
Branch
------
master
Details
-------
http://git.postgresql.org/pg/commitdiff/9c7f5229ad68d7e0e4dd149e3f80257893e404d4
Modified Files
--------------
contrib/citext/expected/citext.out | 2 +-
contrib/citext/expected/citext_1.out | 2 +-
contrib/postgres_fdw/expected/postgres_fdw.out | 69 +++---
src/backend/commands/explain.c | 17 ++
src/backend/executor/nodeHashjoin.c | 13 +-
src/backend/executor/nodeMergejoin.c | 56 +++--
src/backend/executor/nodeNestloop.c | 14 +-
src/backend/nodes/copyfuncs.c | 2 +
src/backend/nodes/outfuncs.c | 5 +
src/backend/nodes/readfuncs.c | 2 +
src/backend/optimizer/path/costsize.c | 132 ++++++-----
src/backend/optimizer/path/joinpath.c | 68 ++++--
src/backend/optimizer/plan/analyzejoins.c | 173 ++++++++++++++
src/backend/optimizer/plan/createplan.c | 31 ++-
src/backend/optimizer/util/pathnode.c | 32 +--
src/backend/optimizer/util/relnode.c | 8 +-
src/include/nodes/execnodes.h | 4 +
src/include/nodes/plannodes.h | 8 +
src/include/nodes/relation.h | 41 +++-
src/include/optimizer/cost.h | 16 +-
src/include/optimizer/pathnode.h | 8 +-
src/include/optimizer/planmain.h | 3 +
src/test/regress/expected/aggregates.out | 32 +--
src/test/regress/expected/equivclass.out | 22 +-
src/test/regress/expected/join.out | 309 ++++++++++++++++++++++++-
src/test/regress/sql/join.sql | 124 ++++++++++
26 files changed, 987 insertions(+), 206 deletions(-)