Implement mxid_age() to compute multi-xid age

Report by Josh Berkus

doc/src/sgml/maintenance.sgml

@@ -640,7 +640,12 @@ HINT:  Stop the postmaster and vacuum that database in single-user mode.
      possible multixact ID still appearing in any tuple of that table.
      If this value is older than
      <xref linkend="guc-vacuum-multixact-freeze-table-age">, a whole-table
-     scan is forced.  Whole-table <command>VACUUM</> scans, regardless of
+     scan is forced.  <function>mxid_age()</> can be used on
+     <structname>pg_class</>.<structfield>relminmxid</> to find its age.
+    </para>
+
+    <para>
+     Whole-table <command>VACUUM</> scans, regardless of
      what causes them, enable advancing the value for that table.
      Eventually, as all tables in all databases are scanned and their
      oldest multixact values are advanced, on-disk storage for older

src/backend/utils/adt/xid.c

@@ -16,6 +16,7 @@
 
 #include <limits.h>
 
+#include "access/multixact.h"
 #include "access/transam.h"
 #include "access/xact.h"
 #include "libpq/pqformat.h"
@@ -102,6 +103,21 @@ xid_age(PG_FUNCTION_ARGS)
 	PG_RETURN_INT32((int32) (now - xid));
 }
 
+/*
+ *		mxid_age			- compute age of a multi XID (relative to latest stable mxid)
+ */
+Datum
+mxid_age(PG_FUNCTION_ARGS)
+{
+	TransactionId xid = PG_GETARG_TRANSACTIONID(0);
+	MultiXactId now = ReadNextMultiXactId();
+
+	if (!MultiXactIdIsValid(xid))
+		PG_RETURN_INT32(INT_MAX);
+
+	PG_RETURN_INT32((int32) (now - xid));
+}
+
 /*
  * xidComparator
  *		qsort comparison function for XIDs

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201409091
+#define CATALOG_VERSION_NO	201409101
 
 #endif

src/include/catalog/pg_proc.h

@@ -1279,6 +1279,8 @@ DATA(insert OID = 1180 (  abstime		   PGNSP PGUID 12 1 0 0 0 f f f f t f i 1 0 7
 DESCR("convert timestamp with time zone to abstime");
 DATA(insert OID = 1181 (  age			   PGNSP PGUID 12 1 0 0 0 f f f f t f s 1 0 23 "28" _null_ _null_ _null_ _null_ xid_age _null_ _null_ _null_ ));
 DESCR("age of a transaction ID, in transactions before current transaction");
+DATA(insert OID = 3939 (  mxid_age		   PGNSP PGUID 12 1 0 0 0 f f f f t f s 1 0 23 "28" _null_ _null_ _null_ _null_ mxid_age _null_ _null_ _null_ ));
+DESCR("age of a multi-transaction ID, in multi-transactions before current multi-transaction");
 
 DATA(insert OID = 1188 (  timestamptz_mi   PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 1186 "1184 1184" _null_ _null_ _null_ _null_ timestamp_mi _null_ _null_ _null_ ));
 DATA(insert OID = 1189 (  timestamptz_pl_interval PGNSP PGUID 12 1 0 0 0 f f f f t f s 2 0 1184 "1184 1186" _null_ _null_ _null_ _null_ timestamptz_pl_interval _null_ _null_ _null_ ));

src/include/utils/builtins.h

@@ -845,6 +845,7 @@ extern Datum xidrecv(PG_FUNCTION_ARGS);
 extern Datum xidsend(PG_FUNCTION_ARGS);
 extern Datum xideq(PG_FUNCTION_ARGS);
 extern Datum xid_age(PG_FUNCTION_ARGS);
+extern Datum mxid_age(PG_FUNCTION_ARGS);
 extern int	xidComparator(const void *arg1, const void *arg2);
 extern Datum cidin(PG_FUNCTION_ARGS);
 extern Datum cidout(PG_FUNCTION_ARGS);