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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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11
src/backend/executor/execCurrent.c
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@ -91,21 +91,22 @@ execCurrentOf(CurrentOfExpr *cexpr,
* the other code can't, while the non-FOR-UPDATE case allows use of WHERE
* CURRENT OF with an insensitive cursor.
*/
if (queryDesc->estate->es_rowMarks)
if (queryDesc->estate->es_rowmarks)
{
ExecRowMark *erm;
ListCell *lc;
Index i;
/*
* Here, the query must have exactly one FOR UPDATE/SHARE reference to
* the target table, and we dig the ctid info out of that.
*/
erm = NULL;
foreach(lc, queryDesc->estate->es_rowMarks)
for (i = 0; i < queryDesc->estate->es_range_table_size; i++)
{
ExecRowMark *thiserm = (ExecRowMark *) lfirst(lc);
ExecRowMark *thiserm = queryDesc->estate->es_rowmarks[i];
if (!RowMarkRequiresRowShareLock(thiserm->markType))
if (thiserm == NULL ||
!RowMarkRequiresRowShareLock(thiserm->markType))
continue; /* ignore non-FOR UPDATE/SHARE items */
if (thiserm->relid == table_oid)