Expose some information about backend subxact status.

A new function pg_stat_get_backend_subxact() can be used to get
information about the number of subtransactions in the cache of
a particular backend and whether that cache has overflowed. This
can be useful for tracking down performance problems that can
result from overflowed snapshots.

Dilip Kumar, reviewed by Zhihong Yu, Nikolay Samokhvalov,
Justin Pryzby, Nathan Bossart, Ashutosh Sharma, Julien
Rouhaud. Additional design comments from Andres Freund,
Tom Lane, Bruce Momjian, and David G. Johnston.

Discussion: http://postgr.es/m/CAFiTN-ut0uwkRJDQJeDPXpVyTWD46m3gt3JDToE02hTfONEN=Q@mail.gmail.com

doc/src/sgml/monitoring.sgml

@@ -5671,6 +5671,24 @@ FROM pg_stat_get_backend_idset() AS backendid;
        </para></entry>
       </row>
 
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_subxact</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_subxact</function> ( <type>integer</type> )
+        <returnvalue>record</returnvalue>
+       </para>
+       <para>
+        Returns a record of information about the subtransactions of the
+        backend with the specified ID.
+        The fields returned are <parameter>subxact_count</parameter>, which
+        is the number of subtransactions in the backend's subtransaction cache,
+        and <parameter>subxact_overflow</parameter>, which indicates whether
+        the backend's subtransaction cache is overflowed or not.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>

src/backend/storage/ipc/sinvaladt.c

@@ -404,17 +404,20 @@ BackendIdGetProc(int backendID)
 
 /*
  * BackendIdGetTransactionIds
- *		Get the xid and xmin of the backend. The result may be out of date
- *		arbitrarily quickly, so the caller must be careful about how this
- *		information is used.
+ *		Get the xid, xmin, nsubxid and overflow status of the backend. The
+ *		result may be out of date arbitrarily quickly, so the caller must be
+ *		careful about how this information is used.
  */
 void
-BackendIdGetTransactionIds(int backendID, TransactionId *xid, TransactionId *xmin)
+BackendIdGetTransactionIds(int backendID, TransactionId *xid,
+						   TransactionId *xmin, int *nsubxid, bool *overflowed)
 {
 	SISeg	   *segP = shmInvalBuffer;
 
 	*xid = InvalidTransactionId;
 	*xmin = InvalidTransactionId;
+	*nsubxid = 0;
+	*overflowed = false;
 
 	/* Need to lock out additions/removals of backends */
 	LWLockAcquire(SInvalWriteLock, LW_SHARED);
@@ -428,6 +431,8 @@ BackendIdGetTransactionIds(int backendID, TransactionId *xid, TransactionId *xmi
 		{
 			*xid = proc->xid;
 			*xmin = proc->xmin;
+			*nsubxid = proc->subxidStatus.count;
+			*overflowed = proc->subxidStatus.overflowed;
 		}
 	}
 

src/backend/utils/activity/backend_status.c

@@ -855,7 +855,9 @@ pgstat_read_current_status(void)
 			localentry->backend_id = i;
 			BackendIdGetTransactionIds(i,
 									   &localentry->backend_xid,
-									   &localentry->backend_xmin);
+									   &localentry->backend_xmin,
+									   &localentry->backend_subxact_count,
+									   &localentry->backend_subxact_overflowed);
 
 			localentry++;
 			localappname += NAMEDATALEN;

src/backend/utils/adt/pgstatfuncs.c

@@ -687,6 +687,44 @@ pg_stat_get_backend_userid(PG_FUNCTION_ARGS)
 	PG_RETURN_OID(beentry->st_userid);
 }
 
+Datum
+pg_stat_get_backend_subxact(PG_FUNCTION_ARGS)
+{
+#define PG_STAT_GET_SUBXACT_COLS	2
+	TupleDesc	tupdesc;
+	Datum		values[PG_STAT_GET_SUBXACT_COLS];
+	bool		nulls[PG_STAT_GET_SUBXACT_COLS];
+	int32		beid = PG_GETARG_INT32(0);
+	LocalPgBackendStatus *local_beentry;
+
+	/* Initialise values and NULL flags arrays */
+	MemSet(values, 0, sizeof(values));
+	MemSet(nulls, 0, sizeof(nulls));
+
+	/* Initialise attributes information in the tuple descriptor */
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_SUBXACT_COLS);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "subxact_count",
+					   INT4OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "subxact_overflow",
+					   BOOLOID, -1, 0);
+
+	BlessTupleDesc(tupdesc);
+
+	if ((local_beentry = pgstat_fetch_stat_local_beentry(beid)) != NULL)
+	{
+		/* Fill values and NULLs */
+		values[0] = Int32GetDatum(local_beentry->backend_subxact_count);
+		values[1] = BoolGetDatum(local_beentry->backend_subxact_overflowed);
+	}
+	else
+	{
+		nulls[0] = true;
+		nulls[1] = true;
+	}
+
+	/* Returns the record as Datum */
+	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
+}
 
 Datum
 pg_stat_get_backend_activity(PG_FUNCTION_ARGS)

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202212131
+#define CATALOG_VERSION_NO	202212191
 
 #endif

src/include/catalog/pg_proc.dat

@@ -5441,6 +5441,13 @@
   proname => 'pg_stat_get_backend_dbid', provolatile => 's', proparallel => 'r',
   prorettype => 'oid', proargtypes => 'int4',
   prosrc => 'pg_stat_get_backend_dbid' },
+{ oid => '6107', descr => 'statistics: get subtransaction status of backend',
+  proname => 'pg_stat_get_backend_subxact', provolatile => 's', proparallel => 'r',
+  prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int4,bool}',
+  proargmodes => '{i,o,o}',
+  proargnames => '{bid,subxact_count,subxact_overflowed}',
+  prosrc => 'pg_stat_get_backend_subxact' },
 { oid => '1939', descr => 'statistics: user ID of backend',
   proname => 'pg_stat_get_backend_userid', provolatile => 's',
   proparallel => 'r', prorettype => 'oid', proargtypes => 'int4',

src/include/storage/sinvaladt.h

@@ -32,7 +32,9 @@ extern Size SInvalShmemSize(void);
 extern void CreateSharedInvalidationState(void);
 extern void SharedInvalBackendInit(bool sendOnly);
 extern PGPROC *BackendIdGetProc(int backendID);
-extern void BackendIdGetTransactionIds(int backendID, TransactionId *xid, TransactionId *xmin);
+extern void BackendIdGetTransactionIds(int backendID, TransactionId *xid,
+									   TransactionId *xmin, int *nsubxid,
+									   bool *overflowed);
 
 extern void SIInsertDataEntries(const SharedInvalidationMessage *data, int n);
 extern int	SIGetDataEntries(SharedInvalidationMessage *data, int datasize);

src/include/utils/backend_status.h

@@ -266,6 +266,17 @@ typedef struct LocalPgBackendStatus
 	 * not.
 	 */
 	TransactionId backend_xmin;
+
+	/*
+	 * Number of cached subtransactions in the current session.
+	 */
+	int	backend_subxact_count;
+
+	/*
+	 * The number of subtransactions in the current session exceeded the cached
+	 * subtransaction limit.
+	 */
+	bool backend_subxact_overflowed;
 } LocalPgBackendStatus;