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Commitcb8e015

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Code review for nodeGatherMerge.c.
Comment the fields of GatherMergeState, and organize them a bit moresensibly. Comment GMReaderTupleBuffer more usefully too. Improveassorted other comments that were obsolete or just not very good English.Get rid of the use of a GMReaderTupleBuffer for the leader process;that was confusing, since only the "done" field was used, and thatin a way redundant with need_to_scan_locally.In gather_merge_init, avoid calling load_tuple_array foralready-known-exhausted workers. I'm not sure if there's a live bug there,but the case is unlikely to be well tested due to timing considerations.Remove some useless code, such as duplicating the tts_isempty test done byTupIsNull.Remove useless initialization of ps.qual, replacing that with an assertionthat we have no qual to check. (If we did, the code would fail to checkit.)Avoid applying heap_copytuple to a null tuple. While that fails to crash,it's confusing and it makes the code less legible not more so IMO.Propagate a couple of these changes into nodeGather.c, as well.Back-patch to v10, partly because of the possibility that thegather_merge_init change is fixing a live bug, but mostly to keepthe branches in sync to ease future bug fixes.
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‎src/backend/executor/nodeGather.c

Lines changed: 12 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -71,6 +71,8 @@ ExecInitGather(Gather *node, EState *estate, int eflags)
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gatherstate->ps.plan= (Plan*)node;
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gatherstate->ps.state=estate;
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gatherstate->ps.ExecProcNode=ExecGather;
74+
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gatherstate->initialized= false;
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gatherstate->need_to_scan_locally= !node->single_copy;
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/*
@@ -81,10 +83,10 @@ ExecInitGather(Gather *node, EState *estate, int eflags)
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ExecAssignExprContext(estate,&gatherstate->ps);
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/*
84-
* initialize child expressions
86+
* Gather doesn't support checking a qual (it's always more efficient to
87+
* do it in the child node).
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*/
86-
gatherstate->ps.qual=
87-
ExecInitQual(node->plan.qual, (PlanState*)gatherstate);
89+
Assert(!node->plan.qual);
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/*
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* tuple table initialization
@@ -167,15 +169,16 @@ ExecGather(PlanState *pstate)
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*/
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pcxt=node->pei->pcxt;
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LaunchParallelWorkers(pcxt);
172+
/* We save # workers launched for the benefit of EXPLAIN */
170173
node->nworkers_launched=pcxt->nworkers_launched;
174+
node->nreaders=0;
175+
node->nextreader=0;
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/* Set up tuple queue readers to read the results. */
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if (pcxt->nworkers_launched>0)
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{
175-
node->nreaders=0;
176-
node->nextreader=0;
177-
node->reader=
178-
palloc(pcxt->nworkers_launched*sizeof(TupleQueueReader*));
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node->reader=palloc(pcxt->nworkers_launched*
181+
sizeof(TupleQueueReader*));
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180183
for (i=0;i<pcxt->nworkers_launched;++i)
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{
@@ -314,8 +317,8 @@ gather_readnext(GatherState *gatherstate)
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tup=TupleQueueReaderNext(reader, true,&readerdone);
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/*
317-
* If this reader is done, remove it. If all readers are done, clean
318-
* up remaining worker state.
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* If this reader is done, remove it, and collapse the array. If all
321+
*readers are done, cleanup remaining worker state.
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*/
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if (readerdone)
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{

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