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Commitbc7fcab

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Read from the same worker repeatedly until it returns no tuple.
The original coding read tuples from workers in round-robin fashion,but performance testing shows that it works much better to read enoughto empty one queue before moving on to the next. I believe thereason for this is that, with the old approach, we could easily wakeup a worker repeatedly to write only one new tuple into the shm_mqeach time. With this approach, by the time the process gets scheduled,it has a decent chance of being able to fill the entire buffer inone go.Patch by me. Dilip Kumar helped with performance testing.
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‎src/backend/executor/nodeGather.c

Lines changed: 10 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -359,14 +359,20 @@ gather_readnext(GatherState *gatherstate)
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continue;
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}
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/* Advance nextreader pointer in round-robin fashion. */
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gatherstate->nextreader=
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(gatherstate->nextreader+1) %gatherstate->nreaders;
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/* If we got a tuple, return it. */
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if (tup)
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returntup;
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/*
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* Advance nextreader pointer in round-robin fashion. Note that we
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* only reach this code if we weren't able to get a tuple from the
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* current worker. We used to advance the nextreader pointer after
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* every tuple, but it turns out to be much more efficient to keep
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* reading from the same queue until that would require blocking.
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*/
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gatherstate->nextreader=
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(gatherstate->nextreader+1) %gatherstate->nreaders;
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/* Have we visited every TupleQueueReader? */
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if (gatherstate->nextreader==waitpos)
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{

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