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Avoid O(N^2) cost in ExecFindRowMark().

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If there are many ExecRowMark structs, we spent O(N^2) time in
ExecFindRowMark during executor startup.  Once upon a time this was
not of great concern, but the addition of native partitioning has
squeezed out enough other costs that this can become the dominant
overhead in some use-cases for tables with many partitions.

To fix, simply replace that List data structure with an array.

This adds a little bit of cost to execCurrentOf(), but not much,
and anyway that code path is neither of large importance nor very
efficient now.  If we ever decide it is a bottleneck, constructing a
hash table for lookup-by-tableoid would likely be the thing to do.

Per complaint from Amit Langote, though this is different from
his fix proposal.

Discussion: https://postgr.es/m/468c85d9-540e-66a2-1dde-fec2b741e688@lab.ntt.co.jp
This commit is contained in:
Tom Lane
2018-10-08 10:41:34 -04:00
parent eee01d606e
commit f9eb7c14b0
4 changed files with 84 additions and 69 deletions
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9
src/backend/executor/execUtils.c
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@ -113,6 +113,7 @@ CreateExecutorState(void)
estate->es_range_table_array = NULL;
estate->es_range_table_size = 0;
estate->es_relations = NULL;
estate->es_rowmarks = NULL;
estate->es_plannedstmt = NULL;
estate->es_junkFilter = NULL;
@ -142,8 +143,6 @@ CreateExecutorState(void)
estate->es_tupleTable = NIL;
estate->es_rowMarks = NIL;
estate->es_processed = 0;
estate->es_lastoid = InvalidOid;
@ -709,6 +708,12 @@ ExecInitRangeTable(EState *estate, List *rangeTable)
*/
estate->es_relations = (Relation *)
palloc0(estate->es_range_table_size * sizeof(Relation));
/*
* es_rowmarks is also parallel to the es_range_table_array, but it's
* allocated only if needed.
*/
estate->es_rowmarks = NULL;
}
/*