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Commit6a9fc75

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Avoid unnecessary shm writes in Parallel Hash Join.
Currently, Parallel Hash Join cannot be used for full/right joins,so there is no point in setting the match flag. It turns out thatthe cache coherence traffic generated by those writes slows downlarge systems running many-core joins, so let's stop doing that.In future, if we need to use match bits in parallel joins, we mightwant to consider setting them only if not already set.Back-patch to 11, where Parallel Hash Join arrived.Reported-by: Deng, GangDiscussion:https://postgr.es/m/0F44E799048C4849BAE4B91012DB910462E9897A%40SHSMSX103.ccr.corp.intel.com
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‎src/backend/executor/nodeHashjoin.c

Lines changed: 20 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -447,7 +447,26 @@ ExecHashJoinImpl(PlanState *pstate, bool parallel)
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if (joinqual==NULL||ExecQual(joinqual,econtext))
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{
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node->hj_MatchedOuter= true;
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HeapTupleHeaderSetMatch(HJTUPLE_MINTUPLE(node->hj_CurTuple));
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if (parallel)
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{
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/*
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* Full/right outer joins are currently not supported
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* for parallel joins, so we don't need to set the
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* match bit. Experiments show that it's worth
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* avoiding the shared memory traffic on large
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* systems.
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*/
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Assert(!HJ_FILL_INNER(node));
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}
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else
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{
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/*
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* This is really only needed if HJ_FILL_INNER(node),
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* but we'll avoid the branch and just set it always.
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*/
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HeapTupleHeaderSetMatch(HJTUPLE_MINTUPLE(node->hj_CurTuple));
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}
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/* In an antijoin, we never return a matched tuple */
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if (node->js.jointype==JOIN_ANTI)

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