Hi all, One obstacle to the move of pg_stat_statements to the shmem pgstats that we have in core is that there is no easy way to track the total number of entries that are stored in the shared hash table of pgstats for variable-sized stats kinds.
PGSS currently hash_get_num_entries() as a cheap way to get the number of entries stored in its hash table, but moving PGSS to the in-core stats facility means that we need a different approach to count these entries. I have looked at what can be done, and finished with a rather simple approach, as we have only one code path when a new entry is inserted, and one code path when an entry is removed when the refcount of an entry reaches 0 (includes resets, drops for kind matches, etc.). The counters are stored in a uint64 atomic, one for each stats kind in PgStat_ShmemControl, set only if the option is enabled. I am wondering how much protection we want for the reads, and chose the lightest "lossy" approach, leaving to stats kind what they want to do when deallocating entries. Hence, a stats kind may want to protect the entry deallocations with an extra lock, but that's not required either depending on how aggressive removals should happen. Please find attached that adds an option for variable-sized stats kinds given these the possibility to track the number of entries in the shared hash table. The option is named track_counts. The patch has some coverage added in the test module injection_points, in its TAP test. Thoughts are welcome. -- Michael
From 0c924a72f586c385f6ab1a22174b9c3b1cf2dd08 Mon Sep 17 00:00:00 2001
From: Michael Paquier <mich...@paquier.xyz>
Date: Fri, 12 Sep 2025 16:09:51 +0900
Subject: [PATCH 1/2] Add support for entry counting in pgstats
Stats kinds can set an option call track_counts, that will make pgstats
track the number of entries that exist in the shared hashtable.
As there is only one code path where a new entry is added, and one code
path where entries are removed, the count tracking is rather
straight-forward. Reads of these counters are optimistic, and may
change across two calls.
A use case of this facility is PGSS, where we need to be able to cap the
number of entries that would be stored in the shared hashtable, based on
its "max" GUC.
---
src/include/utils/pgstat_internal.h | 26 +++++++++++++
src/backend/utils/activity/pgstat.c | 6 +++
src/backend/utils/activity/pgstat_shmem.c | 47 +++++++++++++++++------
3 files changed, 67 insertions(+), 12 deletions(-)
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 6cf00008f633..8762a06081f8 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -216,6 +216,12 @@ typedef struct PgStat_KindInfo
/* Should stats be written to the on-disk stats file? */
bool write_to_file:1;
+ /*
+ * Should the number of entries be tracked? For variable-numbered stats,
+ * to update its PgStat_ShmemControl.entry_counts.
+ */
+ bool track_counts:1;
+
/*
* The size of an entry in the shared stats hash table (pointed to by
* PgStatShared_HashEntry->body). For fixed-numbered statistics, this is
@@ -485,6 +491,15 @@ typedef struct PgStat_ShmemControl
*/
pg_atomic_uint64 gc_request_count;
+ /*
+ * Counters for the number of entries associated to a single stats kind
+ * that uses variable-numbered objects stored in the shared hash table.
+ * These counters can be enabled on a per-kind basis, when track_counts is
+ * set. This counter is incremented each time a new entry is created (not
+ * when reused), and each time an entry is dropped.
+ */
+ pg_atomic_uint64 entry_counts[PGSTAT_KIND_MAX];
+
/*
* Stats data for fixed-numbered objects.
*/
@@ -910,6 +925,17 @@ pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common *entry)
return ((char *) (entry)) + off;
}
+/*
+ * Returns the number of entries counted for a stats kind.
+ */
+static inline uint64
+pgstat_get_entry_count(PgStat_Kind kind)
+{
+ Assert(pgstat_get_kind_info(kind)->track_counts);
+
+ return pg_atomic_read_u64(&pgStatLocal.shmem->entry_counts[kind - 1]);
+}
+
/*
* Returns a pointer to the shared memory area of custom stats for
* fixed-numbered statistics.
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index f8e91484e36b..f6784ad6cd15 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -1490,6 +1490,12 @@ pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
ereport(ERROR,
(errmsg("custom cumulative statistics property is invalid"),
errhint("Custom cumulative statistics require a shared memory size for fixed-numbered objects.")));
+ if (kind_info->track_counts)
+ {
+ ereport(ERROR,
+ (errmsg("custom cumulative statistics property is invalid"),
+ errhint("Custom cumulative statistics cannot use counter tracking for fixed-numbered objects.")));
+ }
}
/*
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 9dc3212f7dd0..141ab10031cd 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -210,27 +210,35 @@ StatsShmemInit(void)
pg_atomic_init_u64(&ctl->gc_request_count, 1);
- /* initialize fixed-numbered stats */
+ /* Do the per-kind initialization */
for (PgStat_Kind kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
{
const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
char *ptr;
- if (!kind_info || !kind_info->fixed_amount)
+ if (!kind_info)
continue;
- if (pgstat_is_kind_builtin(kind))
- ptr = ((char *) ctl) + kind_info->shared_ctl_off;
- else
+ /* initialize counter tracking */
+ if (kind_info->track_counts)
+ pg_atomic_init_u64(&ctl->entry_counts[kind - 1], 0);
+
+ /* initialize fixed-numbered stats */
+ if (kind_info->fixed_amount)
{
- int idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+ if (pgstat_is_kind_builtin(kind))
+ ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+ else
+ {
+ int idx = kind - PGSTAT_KIND_CUSTOM_MIN;
- Assert(kind_info->shared_size != 0);
- ctl->custom_data[idx] = ShmemAlloc(kind_info->shared_size);
- ptr = ctl->custom_data[idx];
+ Assert(kind_info->shared_size != 0);
+ ctl->custom_data[idx] = ShmemAlloc(kind_info->shared_size);
+ ptr = ctl->custom_data[idx];
+ }
+
+ kind_info->init_shmem_cb(ptr);
}
-
- kind_info->init_shmem_cb(ptr);
}
}
else
@@ -303,6 +311,7 @@ pgstat_init_entry(PgStat_Kind kind,
/* Create new stats entry. */
dsa_pointer chunk;
PgStatShared_Common *shheader;
+ const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
/*
* Initialize refcount to 1, marking it as valid / not dropped. The entry
@@ -319,7 +328,7 @@ pgstat_init_entry(PgStat_Kind kind,
shhashent->dropped = false;
chunk = dsa_allocate_extended(pgStatLocal.dsa,
- pgstat_get_kind_info(kind)->shared_size,
+ kind_info->shared_size,
DSA_ALLOC_ZERO | DSA_ALLOC_NO_OOM);
if (chunk == InvalidDsaPointer)
return NULL;
@@ -330,6 +339,10 @@ pgstat_init_entry(PgStat_Kind kind,
/* Link the new entry from the hash entry. */
shhashent->body = chunk;
+ /* Increment entry count, if required. */
+ if (kind_info->track_counts)
+ pg_atomic_fetch_add_u64(&pgStatLocal.shmem->entry_counts[kind - 1], 1);
+
LWLockInitialize(&shheader->lock, LWTRANCHE_PGSTATS_DATA);
return shheader;
@@ -910,7 +923,17 @@ pgstat_drop_entry_internal(PgStatShared_HashEntry *shent,
/* release refcount marking entry as not dropped */
if (pg_atomic_sub_fetch_u32(&shent->refcount, 1) == 0)
{
+ PgStat_Kind kind = shent->key.kind;
+
pgstat_free_entry(shent, hstat);
+
+ /*
+ * Entry has been dropped with refcount at 0, hence decrement the
+ * entry counter.
+ */
+ if (pgstat_get_kind_info(kind)->track_counts)
+ pg_atomic_sub_fetch_u64(&pgStatLocal.shmem->entry_counts[kind - 1], 1);
+
return true;
}
else
--
2.51.0
From d92077bd939ac252e9dd245fec1e9470668411de Mon Sep 17 00:00:00 2001
From: Michael Paquier <mich...@paquier.xyz>
Date: Fri, 12 Sep 2025 16:12:11 +0900
Subject: [PATCH 2/2] injection_points: Add entry counting
This serves as coverage for the track_counts, as other in-core stats
kinds do not need to cap their maximum.
---
.../injection_points/injection_points--1.0.sql | 10 ++++++++++
src/test/modules/injection_points/injection_stats.c | 12 ++++++++++++
src/test/modules/injection_points/t/001_stats.pl | 12 ++++++++++++
3 files changed, 34 insertions(+)
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index 5f5657b2043c..a7b61fbdfe6f 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -99,6 +99,16 @@ RETURNS bigint
AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
LANGUAGE C STRICT;
+--
+-- injection_points_stats_count()
+--
+-- Return the number of entries stored in the pgstats hash table.
+--
+CREATE FUNCTION injection_points_stats_count()
+RETURNS bigint
+AS 'MODULE_PATHNAME', 'injection_points_stats_count'
+LANGUAGE C STRICT;
+
--
-- injection_points_stats_drop()
--
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
index e3947b23ba57..15ad9883dedb 100644
--- a/src/test/modules/injection_points/injection_stats.c
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -49,6 +49,7 @@ static const PgStat_KindInfo injection_stats = {
.shared_data_len = sizeof(((PgStatShared_InjectionPoint *) 0)->stats),
.pending_size = sizeof(PgStat_StatInjEntry),
.flush_pending_cb = injection_stats_flush_cb,
+ .track_counts = true,
};
/*
@@ -198,6 +199,17 @@ injection_points_stats_numcalls(PG_FUNCTION_ARGS)
PG_RETURN_INT64(entry->numcalls);
}
+/*
+ * SQL function returning the number of entries allocated for injection
+ * points in the shared hashtable of pgstats.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_count);
+Datum
+injection_points_stats_count(PG_FUNCTION_ARGS)
+{
+ PG_RETURN_INT64(pgstat_get_entry_count(PGSTAT_KIND_INJECTION));
+}
+
/* Only used by injection_points_stats_drop() */
static bool
match_inj_entries(PgStatShared_HashEntry *entry, Datum match_data)
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
index 25de5fc46fe4..47ab58d0e9b8 100644
--- a/src/test/modules/injection_points/t/001_stats.pl
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -36,6 +36,9 @@ $node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
my $numcalls = $node->safe_psql('postgres',
"SELECT injection_points_stats_numcalls('stats-notice');");
is($numcalls, '2', 'number of stats calls');
+my $entrycount =
+ $node->safe_psql('postgres', "SELECT injection_points_stats_count();");
+is($entrycount, '1', 'number of entries');
my $fixedstats = $node->safe_psql('postgres',
"SELECT * FROM injection_points_stats_fixed();");
is($fixedstats, '1|0|2|0|0', 'fixed stats after some calls');
@@ -55,6 +58,9 @@ $node->restart;
$numcalls = $node->safe_psql('postgres',
"SELECT injection_points_stats_numcalls('stats-notice');");
is($numcalls, '3', 'number of stats after clean restart');
+$entrycount =
+ $node->safe_psql('postgres', "SELECT injection_points_stats_count();");
+is($entrycount, '1', 'number of entries after clean restart');
$fixedstats = $node->safe_psql('postgres',
"SELECT * FROM injection_points_stats_fixed();");
is($fixedstats, '1|0|2|1|1', 'fixed stats after clean restart');
@@ -65,6 +71,9 @@ $node->start;
$numcalls = $node->safe_psql('postgres',
"SELECT injection_points_stats_numcalls('stats-notice');");
is($numcalls, '', 'number of stats after crash');
+$entrycount =
+ $node->safe_psql('postgres', "SELECT injection_points_stats_count();");
+is($entrycount, '0', 'number of entries after crash');
$fixedstats = $node->safe_psql('postgres',
"SELECT * FROM injection_points_stats_fixed();");
is($fixedstats, '0|0|0|0|0', 'fixed stats after crash');
@@ -81,6 +90,9 @@ $node->safe_psql('postgres', "SELECT injection_points_stats_drop();");
$numcalls = $node->safe_psql('postgres',
"SELECT injection_points_stats_numcalls('stats-notice');");
is($numcalls, '', 'no stats after drop via SQL function');
+$entrycount =
+ $node->safe_psql('postgres', "SELECT injection_points_stats_count();");
+is($entrycount, '0', 'number of entries after drop via SQL function');
# Stop the server, disable the module, then restart. The server
# should be able to come up.
--
2.51.0
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