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Support parallel joins, and make related improvements.

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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.
This commit is contained in:
Robert Haas
2016-01-20 14:29:22 -05:00
parent a7de3dc5c3
commit 45be99f8cd
15 changed files with 875 additions and 119 deletions
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4
src/backend/nodes/outfuncs.c
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@ -1591,6 +1591,8 @@ _outPathInfo(StringInfo str, const Path *node)
else
_outBitmapset(str, NULL);
WRITE_BOOL_FIELD(parallel_aware);
WRITE_BOOL_FIELD(parallel_safe);
WRITE_INT_FIELD(parallel_degree);
WRITE_FLOAT_FIELD(rows, "%.0f");
WRITE_FLOAT_FIELD(startup_cost, "%.2f");
WRITE_FLOAT_FIELD(total_cost, "%.2f");
@ -1768,7 +1770,6 @@ _outGatherPath(StringInfo str, const GatherPath *node)
_outPathInfo(str, (const Path *) node);
WRITE_NODE_FIELD(subpath);
WRITE_INT_FIELD(num_workers);
WRITE_BOOL_FIELD(single_copy);
}
@ -1890,6 +1891,7 @@ _outRelOptInfo(StringInfo str, const RelOptInfo *node)
WRITE_NODE_FIELD(reltargetlist);
WRITE_NODE_FIELD(pathlist);
WRITE_NODE_FIELD(ppilist);
WRITE_NODE_FIELD(partial_pathlist);
WRITE_NODE_FIELD(cheapest_startup_path);
WRITE_NODE_FIELD(cheapest_total_path);
WRITE_NODE_FIELD(cheapest_unique_path);