Support parallel joins, and make related improvements

Enterprise / PostgreSQL - Robert Haas [postgresql.org] - 20 January 2016 13:40 UTC

The core innovation of this patch is the introduction of the concept of a partial path; that is, a path which if executed in parallel will generate a subset of the output rows in each process. Gathering a partial path produces an ordinary (complete) path. This allows us to generate paths for parallel joins by joining a partial path for one side (which at the baserel level is currently always a Partial Seq Scan) to an ordinary path on the other side. This is subject to
various restrictions at present, especially that this strategy seems unlikely to be sensible for merge joins, so only nested loops and hash joins paths are generated.

This also allows an Append node to be pushed below a Gather node in the case of a partitioned table.

Testing revealed that early versions of this patch made poor decisions in some cases, which turned out to be caused by the fact that the original cost model for Parallel Seq Scan wasn't very good. So this patch tries to make some modest improvements in that area.

There is much more to be done in the area of generating good parallel plans in all cases, but this seems like a useful step forward.

Patch by me, reviewed by Dilip Kumar and Amit Kapila.

45be99f Support parallel joins, and make related improvements.
src/backend/executor/execParallel.c | 66 +++---
src/backend/nodes/outfuncs.c | 4 +-
src/backend/optimizer/README | 55 ++++-
src/backend/optimizer/path/allpaths.c | 164 ++++++++++----
src/backend/optimizer/path/costsize.c | 61 ++++--
src/backend/optimizer/path/joinpath.c | 253 +++++++++++++++++++++-
src/backend/optimizer/path/joinrels.c | 3 +-
src/backend/optimizer/plan/createplan.c | 2 +-
src/backend/optimizer/plan/planmain.c | 3 +-
src/backend/optimizer/util/pathnode.c | 361 ++++++++++++++++++++++++++++---
src/backend/optimizer/util/relnode.c | 2 +
src/include/nodes/relation.h | 4 +-
src/include/optimizer/cost.h | 2 +-
src/include/optimizer/pathnode.h | 12 +-
src/include/optimizer/paths.h | 2 +
15 files changed, 875 insertions(+), 119 deletions(-)

Upstream: git.postgresql.org

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