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Commit166f943

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Clarify the logic in a few places in the new balanced merge code.
In selectnewtape(), use 'nOutputTapes' rather than 'nOutputRuns' in thecheck for whether to start a new tape or to append a new run to anexisting tape. Until 'maxTapes' is reached, nOutputTapes is always equalto nOutputRuns, so it doesn't change the logic, but it seems more logicalto compare # of tapes with # of tapes. Also, currently maxTapes is nevermodified after the merging begins, but written this way, the code wouldstill work if it was. (Although the nOutputRuns == nOutputTapes assertionwould need to be removed and using nOutputRuns % nOutputTapes todistribute the runs evenly across the tapes wouldn't do a good jobanymore).Similarly in mergeruns(), change to USEMEM(state->tape_buffer_mem) toaccount for the memory used for tape buffers. It's equal to availMemcurrently, but tape_buffer_mem is more direct and future-proof. Forexample, if we changed the logic to only allocate half of the remainingmemory to tape buffers, USEMEM(state->tape_buffer_mem) would still becorrect.Coverity complained about these. Hopefully this patch helps it tounderstand the logic better. Thanks to Tom Lane for initial analysis.
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‎src/backend/utils/sort/tuplesort.c

Lines changed: 9 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -2773,13 +2773,19 @@ inittapestate(Tuplesortstate *state, int maxTapes)
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staticvoid
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selectnewtape(Tuplesortstate*state)
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{
2776-
if (state->nOutputRuns<state->maxTapes)
2776+
/*
2777+
* At the beginning of each merge pass, nOutputTapes and nOutputRuns are
2778+
* both zero. On each call, we create a new output tape to hold the next
2779+
* run, until maxTapes is reached. After that, we assign new runs to the
2780+
* existing tapes in a round robin fashion.
2781+
*/
2782+
if (state->nOutputTapes<state->maxTapes)
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{
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/* Create a new tape to hold the next run */
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Assert(state->outputTapes[state->nOutputRuns]==NULL);
27802786
Assert(state->nOutputRuns==state->nOutputTapes);
27812787
state->destTape=LogicalTapeCreate(state->tapeset);
2782-
state->outputTapes[state->nOutputRuns]=state->destTape;
2788+
state->outputTapes[state->nOutputTapes]=state->destTape;
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state->nOutputTapes++;
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state->nOutputRuns++;
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}
@@ -2908,7 +2914,7 @@ mergeruns(Tuplesortstate *state)
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* divide this memory between the input and output tapes in the pass.
29092915
*/
29102916
state->tape_buffer_mem=state->availMem;
2911-
USEMEM(state,state->availMem);
2917+
USEMEM(state,state->tape_buffer_mem);
29122918
#ifdefTRACE_SORT
29132919
if (trace_sort)
29142920
elog(LOG,"worker %d using %zu KB of memory for tape buffers",

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