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Invent qsort_interruptible().

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Justin Pryzby reported that some scenarios could cause gathering
of extended statistics to spend many seconds in an un-cancelable
qsort() operation.  To fix, invent qsort_interruptible(), which is
just like qsort_arg() except that it will also do CHECK_FOR_INTERRUPTS
every so often.  This bloats the backend by a couple of kB, which
seems like a good investment.  (We considered just enabling
CHECK_FOR_INTERRUPTS in the existing qsort and qsort_arg functions,
but there are some callers for which that'd demonstrably be unsafe.
Opt-in seems like a better way.)

For now, just apply qsort_interruptible() in statistics collection.
There's probably more places where it could be useful, but we can
always change other call sites as we find problems.

Back-patch to v14.  Before that we didn't have extended stats on
expressions, so that the problem was less severe.  Also, this patch
depends on the sort_template infrastructure introduced in v14.

Tom Lane and Justin Pryzby

Discussion: https://postgr.es/m/20220509000108.GQ28830@telsasoft.com
This commit is contained in:
Tom Lane
2022-07-12 16:30:36 -04:00
parent 920072339f
commit e64cdab003
10 changed files with 80 additions and 55 deletions
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14
src/backend/statistics/mcv.c
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@@ -404,7 +404,7 @@ count_distinct_groups(int numrows, SortItem *items, MultiSortSupport mss)
* order.
*/
static int
compare_sort_item_count(const void *a, const void *b)
compare_sort_item_count(const void *a, const void *b, void *arg)
{
SortItem *ia = (SortItem *) a;
SortItem *ib = (SortItem *) b;
@@ -457,8 +457,8 @@ build_distinct_groups(int numrows, SortItem *items, MultiSortSupport mss,
Assert(j + 1 == ngroups);
/* Sort the distinct groups by frequency (in descending order). */
pg_qsort((void *) groups, ngroups, sizeof(SortItem),
compare_sort_item_count);
qsort_interruptible((void *) groups, ngroups, sizeof(SortItem),
compare_sort_item_count, NULL);
*ndistinct = ngroups;
return groups;
@@ -528,8 +528,8 @@ build_column_frequencies(SortItem *groups, int ngroups,
}
/* sort the values, deduplicate */
qsort_arg((void *) result[dim], ngroups, sizeof(SortItem),
sort_item_compare, ssup);
qsort_interruptible((void *) result[dim], ngroups, sizeof(SortItem),
sort_item_compare, ssup);
/*
* Identify distinct values, compute frequency (there might be
@@ -696,8 +696,8 @@ statext_mcv_serialize(MCVList *mcvlist, VacAttrStats **stats)
PrepareSortSupportFromOrderingOp(typentry->lt_opr, &ssup[dim]);
qsort_arg(values[dim], counts[dim], sizeof(Datum),
compare_scalars_simple, &ssup[dim]);
qsort_interruptible(values[dim], counts[dim], sizeof(Datum),
compare_scalars_simple, &ssup[dim]);
/*
* Walk through the array and eliminate duplicate values, but keep the