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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 d8759a40bf
commit 12c99c8844
10 changed files with 80 additions and 55 deletions
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25
src/backend/commands/analyze.c
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@ -100,7 +100,7 @@ static VacAttrStats *examine_attribute(Relation onerel, int attnum,
static int acquire_sample_rows(Relation onerel, int elevel,
HeapTuple *rows, int targrows,
double *totalrows, double *totaldeadrows);
static int compare_rows(const void *a, const void *b);
static int compare_rows(const void *a, const void *b, void *arg);
static int acquire_inherited_sample_rows(Relation onerel, int elevel,
HeapTuple *rows, int targrows,
double *totalrows, double *totaldeadrows);
@ -1306,7 +1306,8 @@ acquire_sample_rows(Relation onerel, int elevel,
* tuples are already sorted.
*/
if (numrows == targrows)
qsort((void *) rows, numrows, sizeof(HeapTuple), compare_rows);
qsort_interruptible((void *) rows, numrows, sizeof(HeapTuple),
compare_rows, NULL);
/*
* Estimate total numbers of live and dead rows in relation, extrapolating
@ -1342,10 +1343,10 @@ acquire_sample_rows(Relation onerel, int elevel,
}
/*
* qsort comparator for sorting rows[] array
* Comparator for sorting rows[] array
*/
static int
compare_rows(const void *a, const void *b)
compare_rows(const void *a, const void *b, void *arg)
{
HeapTuple ha = *(const HeapTuple *) a;
HeapTuple hb = *(const HeapTuple *) b;
@ -1839,7 +1840,7 @@ static void compute_scalar_stats(VacAttrStatsP stats,
int samplerows,
double totalrows);
static int compare_scalars(const void *a, const void *b, void *arg);
static int compare_mcvs(const void *a, const void *b);
static int compare_mcvs(const void *a, const void *b, void *arg);
static int analyze_mcv_list(int *mcv_counts,
int num_mcv,
double stadistinct,
@ -2476,8 +2477,8 @@ compute_scalar_stats(VacAttrStatsP stats,
/* Sort the collected values */
cxt.ssup = &ssup;
cxt.tupnoLink = tupnoLink;
qsort_arg((void *) values, values_cnt, sizeof(ScalarItem),
compare_scalars, (void *) &cxt);
qsort_interruptible((void *) values, values_cnt, sizeof(ScalarItem),
compare_scalars, (void *) &cxt);
/*
* Now scan the values in order, find the most common ones, and also
@ -2721,8 +2722,8 @@ compute_scalar_stats(VacAttrStatsP stats,
deltafrac;
/* Sort the MCV items into position order to speed next loop */
qsort((void *) track, num_mcv,
sizeof(ScalarMCVItem), compare_mcvs);
qsort_interruptible((void *) track, num_mcv, sizeof(ScalarMCVItem),
compare_mcvs, NULL);
/*
* Collapse out the MCV items from the values[] array.
@ -2885,7 +2886,7 @@ compute_scalar_stats(VacAttrStatsP stats,
}
/*
* qsort_arg comparator for sorting ScalarItems
* Comparator for sorting ScalarItems
*
* Aside from sorting the items, we update the tupnoLink[] array
* whenever two ScalarItems are found to contain equal datums. The array
@ -2922,10 +2923,10 @@ compare_scalars(const void *a, const void *b, void *arg)
}
/*
* qsort comparator for sorting ScalarMCVItems by position
* Comparator for sorting ScalarMCVItems by position
*/
static int
compare_mcvs(const void *a, const void *b)
compare_mcvs(const void *a, const void *b, void *arg)
{
int da = ((const ScalarMCVItem *) a)->first;
int db = ((const ScalarMCVItem *) b)->first;