Invent qsort_interruptible().

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

src/backend/commands/analyze.c

@@ -99,7 +99,7 @@ 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);
@@ -1324,7 +1324,8 @@ 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
@@ -1360,10 +1361,10 @@ 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;
@@ -1857,7 +1858,7 @@ 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,
@@ -2494,7 +2495,7 @@ compute_scalar_stats(VacAttrStatsP stats,
 		/* Sort the collected values */
 		cxt.ssup = &ssup;
 		cxt.tupnoLink = tupnoLink;
-		qsort_arg((void *) values, values_cnt, sizeof(ScalarItem),
+		qsort_interruptible((void *) values, values_cnt, sizeof(ScalarItem),
 							compare_scalars, (void *) &cxt);
 
 		/*
@@ -2739,8 +2740,8 @@ 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.
@@ -2903,7 +2904,7 @@ 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
@@ -2940,10 +2941,10 @@ 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;

src/backend/statistics/extended_stats.c

@@ -1133,7 +1133,7 @@ build_sorted_items(StatsBuildData *data, int *nitems,
 	}
 
 	/* do the sort, using the multi-sort */
-	qsort_arg((void *) items, nrows, sizeof(SortItem),
+	qsort_interruptible((void *) items, nrows, sizeof(SortItem),
 						multi_sort_compare, mss);
 
 	return items;

src/backend/statistics/mcv.c

@@ -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;
@@ -528,7 +528,7 @@ build_column_frequencies(SortItem *groups, int ngroups,
 		}
 
 		/* sort the values, deduplicate */
-		qsort_arg((void *) result[dim], ngroups, sizeof(SortItem),
+		qsort_interruptible((void *) result[dim], ngroups, sizeof(SortItem),
 							sort_item_compare, ssup);
 
 		/*
@@ -695,7 +695,7 @@ statext_mcv_serialize(MCVList *mcvlist, VacAttrStats **stats)
 
 		PrepareSortSupportFromOrderingOp(typentry->lt_opr, &ssup[dim]);
 
-		qsort_arg(values[dim], counts[dim], sizeof(Datum),
+		qsort_interruptible(values[dim], counts[dim], sizeof(Datum),
 							compare_scalars_simple, &ssup[dim]);
 
 		/*

src/backend/statistics/mvdistinct.c

@@ -488,7 +488,7 @@ ndistinct_for_combination(double totalrows, StatsBuildData *data,
 	}
 
 	/* We can sort the array now ... */
-	qsort_arg((void *) items, numrows, sizeof(SortItem),
+	qsort_interruptible((void *) items, numrows, sizeof(SortItem),
 						multi_sort_compare, mss);
 
 	/* ... and count the number of distinct combinations */

src/backend/tsearch/ts_typanalyze.c

@@ -44,8 +44,10 @@ static void prune_lexemes_hashtable(HTAB *lexemes_tab, int b_current);
 static uint32 lexeme_hash(const void *key, Size keysize);
 static int	lexeme_match(const void *key1, const void *key2, Size keysize);
 static int	lexeme_compare(const void *key1, const void *key2);
-static int	trackitem_compare_frequencies_desc(const void *e1, const void *e2);
-static int	trackitem_compare_lexemes(const void *e1, const void *e2);
+static int	trackitem_compare_frequencies_desc(const void *e1, const void *e2,
+											   void *arg);
+static int	trackitem_compare_lexemes(const void *e1, const void *e2,
+									  void *arg);
 
 
 /*
@@ -347,8 +349,8 @@ compute_tsvector_stats(VacAttrStats *stats,
 		 */
 		if (num_mcelem < track_len)
 		{
-			qsort(sort_table, track_len, sizeof(TrackItem *),
-				  trackitem_compare_frequencies_desc);
+			qsort_interruptible(sort_table, track_len, sizeof(TrackItem *),
+								trackitem_compare_frequencies_desc, NULL);
 			/* reset minfreq to the smallest frequency we're keeping */
 			minfreq = sort_table[num_mcelem - 1]->frequency;
 		}
@@ -376,8 +378,8 @@ compute_tsvector_stats(VacAttrStats *stats,
 			 * presorted we can employ binary search for that.  See
 			 * ts_selfuncs.c for a real usage scenario.
 			 */
-			qsort(sort_table, num_mcelem, sizeof(TrackItem *),
-				  trackitem_compare_lexemes);
+			qsort_interruptible(sort_table, num_mcelem, sizeof(TrackItem *),
+								trackitem_compare_lexemes, NULL);
 
 			/* Must copy the target values into anl_context */
 			old_context = MemoryContextSwitchTo(stats->anl_context);
@@ -510,10 +512,10 @@ lexeme_compare(const void *key1, const void *key2)
 }
 
 /*
- *	qsort() comparator for sorting TrackItems on frequencies (descending sort)
+ *	Comparator for sorting TrackItems on frequencies (descending sort)
  */
 static int
-trackitem_compare_frequencies_desc(const void *e1, const void *e2)
+trackitem_compare_frequencies_desc(const void *e1, const void *e2, void *arg)
 {
 	const TrackItem *const *t1 = (const TrackItem *const *) e1;
 	const TrackItem *const *t2 = (const TrackItem *const *) e2;
@@ -522,10 +524,10 @@ trackitem_compare_frequencies_desc(const void *e1, const void *e2)
 }
 
 /*
- *	qsort() comparator for sorting TrackItems on lexemes
+ *	Comparator for sorting TrackItems on lexemes
  */
 static int
-trackitem_compare_lexemes(const void *e1, const void *e2)
+trackitem_compare_lexemes(const void *e1, const void *e2, void *arg)
 {
 	const TrackItem *const *t1 = (const TrackItem *const *) e1;
 	const TrackItem *const *t2 = (const TrackItem *const *) e2;

src/backend/utils/adt/array_typanalyze.c

@@ -86,9 +86,9 @@ static void prune_element_hashtable(HTAB *elements_tab, int b_current);
 static uint32 element_hash(const void *key, Size keysize);
 static int	element_match(const void *key1, const void *key2, Size keysize);
 static int	element_compare(const void *key1, const void *key2);
-static int	trackitem_compare_frequencies_desc(const void *e1, const void *e2);
-static int	trackitem_compare_element(const void *e1, const void *e2);
-static int	countitem_compare_count(const void *e1, const void *e2);
+static int	trackitem_compare_frequencies_desc(const void *e1, const void *e2, void *arg);
+static int	trackitem_compare_element(const void *e1, const void *e2, void *arg);
+static int	countitem_compare_count(const void *e1, const void *e2, void *arg);
 
 
 /*
@@ -502,8 +502,8 @@ compute_array_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc,
 		 */
 		if (num_mcelem < track_len)
 		{
-			qsort(sort_table, track_len, sizeof(TrackItem *),
-				  trackitem_compare_frequencies_desc);
+			qsort_interruptible(sort_table, track_len, sizeof(TrackItem *),
+								trackitem_compare_frequencies_desc, NULL);
 			/* reset minfreq to the smallest frequency we're keeping */
 			minfreq = sort_table[num_mcelem - 1]->frequency;
 		}
@@ -522,8 +522,8 @@ compute_array_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc,
 			 * the element type's default comparison function.  This permits
 			 * fast binary searches in selectivity estimation functions.
 			 */
-			qsort(sort_table, num_mcelem, sizeof(TrackItem *),
-				  trackitem_compare_element);
+			qsort_interruptible(sort_table, num_mcelem, sizeof(TrackItem *),
+								trackitem_compare_element, NULL);
 
 			/* Must copy the target values into anl_context */
 			old_context = MemoryContextSwitchTo(stats->anl_context);
@@ -599,8 +599,9 @@ compute_array_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc,
 			{
 				sorted_count_items[j++] = count_item;
 			}
-			qsort(sorted_count_items, count_items_count,
-				  sizeof(DECountItem *), countitem_compare_count);
+			qsort_interruptible(sorted_count_items, count_items_count,
+								sizeof(DECountItem *),
+								countitem_compare_count, NULL);
 
 			/*
 			 * Prepare to fill stanumbers with the histogram, followed by the
@@ -751,10 +752,10 @@ element_compare(const void *key1, const void *key2)
 }
 
 /*
- * qsort() comparator for sorting TrackItems by frequencies (descending sort)
+ * Comparator for sorting TrackItems by frequencies (descending sort)
  */
 static int
-trackitem_compare_frequencies_desc(const void *e1, const void *e2)
+trackitem_compare_frequencies_desc(const void *e1, const void *e2, void *arg)
 {
 	const TrackItem *const *t1 = (const TrackItem *const *) e1;
 	const TrackItem *const *t2 = (const TrackItem *const *) e2;
@@ -763,10 +764,10 @@ trackitem_compare_frequencies_desc(const void *e1, const void *e2)
 }
 
 /*
- * qsort() comparator for sorting TrackItems by element values
+ * Comparator for sorting TrackItems by element values
  */
 static int
-trackitem_compare_element(const void *e1, const void *e2)
+trackitem_compare_element(const void *e1, const void *e2, void *arg)
 {
 	const TrackItem *const *t1 = (const TrackItem *const *) e1;
 	const TrackItem *const *t2 = (const TrackItem *const *) e2;
@@ -775,10 +776,10 @@ trackitem_compare_element(const void *e1, const void *e2)
 }
 
 /*
- * qsort() comparator for sorting DECountItems by count
+ * Comparator for sorting DECountItems by count
  */
 static int
-countitem_compare_count(const void *e1, const void *e2)
+countitem_compare_count(const void *e1, const void *e2, void *arg)
 {
 	const DECountItem *const *t1 = (const DECountItem *const *) e1;
 	const DECountItem *const *t2 = (const DECountItem *const *) e2;

src/backend/utils/adt/rangetypes_typanalyze.c

@@ -32,7 +32,7 @@
 #include "utils/rangetypes.h"
 #include "utils/multirangetypes.h"
 
-static int	float8_qsort_cmp(const void *a1, const void *a2);
+static int	float8_qsort_cmp(const void *a1, const void *a2, void *arg);
 static int	range_bound_qsort_cmp(const void *a1, const void *a2, void *arg);
 static void compute_range_stats(VacAttrStats *stats,
 								AnalyzeAttrFetchFunc fetchfunc, int samplerows,
@@ -93,7 +93,7 @@ multirange_typanalyze(PG_FUNCTION_ARGS)
  * Comparison function for sorting float8s, used for range lengths.
  */
 static int
-float8_qsort_cmp(const void *a1, const void *a2)
+float8_qsort_cmp(const void *a1, const void *a2, void *arg)
 {
 	const float8 *f1 = (const float8 *) a1;
 	const float8 *f2 = (const float8 *) a2;
@@ -280,9 +280,9 @@ compute_range_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc,
 		if (non_empty_cnt >= 2)
 		{
 			/* Sort bound values */
-			qsort_arg(lowers, non_empty_cnt, sizeof(RangeBound),
+			qsort_interruptible(lowers, non_empty_cnt, sizeof(RangeBound),
 								range_bound_qsort_cmp, typcache);
-			qsort_arg(uppers, non_empty_cnt, sizeof(RangeBound),
+			qsort_interruptible(uppers, non_empty_cnt, sizeof(RangeBound),
 								range_bound_qsort_cmp, typcache);
 
 			num_hist = non_empty_cnt;
@@ -345,7 +345,8 @@ compute_range_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc,
 			 * Ascending sort of range lengths for further filling of
 			 * histogram
 			 */
-			qsort(lengths, non_empty_cnt, sizeof(float8), float8_qsort_cmp);
+			qsort_interruptible(lengths, non_empty_cnt, sizeof(float8),
+								float8_qsort_cmp, NULL);
 
 			num_hist = non_empty_cnt;
 			if (num_hist > num_bins)

src/backend/utils/sort/Makefile

@@ -16,6 +16,7 @@ override CPPFLAGS := -I. -I$(srcdir) $(CPPFLAGS)
 
 OBJS = \
 	logtape.o \
+	qsort_interruptible.o \
 	sharedtuplestore.o \
 	sortsupport.o \
 	tuplesort.o \

src/backend/utils/sort/qsort_interruptible.c

+/*
+ *	qsort_interruptible.c: qsort_arg that includes CHECK_FOR_INTERRUPTS
+ */
+
+#include "postgres.h"
+#include "miscadmin.h"
+
+#define ST_SORT qsort_interruptible
+#define ST_ELEMENT_TYPE_VOID
+#define ST_COMPARATOR_TYPE_NAME qsort_arg_comparator
+#define ST_COMPARE_RUNTIME_POINTER
+#define ST_COMPARE_ARG_TYPE void
+#define ST_SCOPE
+#define ST_DEFINE
+#define ST_CHECK_FOR_INTERRUPTS
+#include "lib/sort_template.h"

src/include/port.h

@@ -508,6 +508,9 @@ typedef int (*qsort_arg_comparator) (const void *a, const void *b, void *arg);
 extern void qsort_arg(void *base, size_t nel, size_t elsize,
 					  qsort_arg_comparator cmp, void *arg);
 
+extern void qsort_interruptible(void *base, size_t nel, size_t elsize,
+								qsort_arg_comparator cmp, void *arg);
+
 extern void *bsearch_arg(const void *key, const void *base,
 						 size_t nmemb, size_t size,
 						 int (*compar) (const void *, const void *, void *),