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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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31
src/backend/utils/adt/array_typanalyze.c
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@@ -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;