Separate multixact freezing parameters from xid's

Previously we were piggybacking on transaction ID parameters to freeze
multixacts; but since there isn't necessarily any relationship between
rates of Xid and multixact consumption, this turns out not to be a good
idea.

Therefore, we now have multixact-specific freezing parameters:

vacuum_multixact_freeze_min_age: when to remove multis as we come across
them in vacuum (default to 5 million, i.e. early in comparison to Xid's
default of 50 million)

vacuum_multixact_freeze_table_age: when to force whole-table scans
instead of scanning only the pages marked as not all visible in
visibility map (default to 150 million, same as for Xids).  Whichever of
both which reaches the 150 million mark earlier will cause a whole-table
scan.

autovacuum_multixact_freeze_max_age: when for cause emergency,
uninterruptible whole-table scans (default to 400 million, double as
that for Xids).  This means there shouldn't be more frequent emergency
vacuuming than previously, unless multixacts are being used very
rapidly.

Backpatch to 9.3 where multixacts were made to persist enough to require
freezing.  To avoid an ABI break in 9.3, VacuumStmt has a couple of
fields in an unnatural place, and StdRdOptions is split in two so that
the newly added fields can go at the end.

Patch by me, reviewed by Robert Haas, with additional input from Andres
Freund and Tom Lane.

src/include/commands/cluster.h

@@ -20,7 +20,8 @@
 
 extern void cluster(ClusterStmt *stmt, bool isTopLevel);
 extern void cluster_rel(Oid tableOid, Oid indexOid, bool recheck,
-			bool verbose, int freeze_min_age, int freeze_table_age);
+			bool verbose, int freeze_min_age, int freeze_table_age,
+			int multixact_freeze_min_age, int multixact_freeze_table_age);
 extern void check_index_is_clusterable(Relation OldHeap, Oid indexOid,
 						   bool recheck, LOCKMODE lockmode);
 extern void mark_index_clustered(Relation rel, Oid indexOid, bool is_internal);

src/include/commands/vacuum.h

@@ -136,6 +136,8 @@ extern PGDLLIMPORT int default_statistics_target;		/* PGDLLIMPORT for
 														 * PostGIS */
 extern int	vacuum_freeze_min_age;
 extern int	vacuum_freeze_table_age;
+extern int	vacuum_multixact_freeze_min_age;
+extern int	vacuum_multixact_freeze_table_age;
 
 
 /* in commands/vacuum.c */
@@ -156,6 +158,8 @@ extern void vac_update_relstats(Relation relation,
 					TransactionId frozenxid,
 					MultiXactId minmulti);
 extern void vacuum_set_xid_limits(int freeze_min_age, int freeze_table_age,
+					  int multixact_freeze_min_age,
+					  int multixact_freeze_table_age,
 					  bool sharedRel,
 					  TransactionId *oldestXmin,
 					  TransactionId *freezeLimit,

src/include/nodes/parsenodes.h

@@ -2432,6 +2432,11 @@ typedef struct VacuumStmt
 	int			freeze_table_age;		/* age at which to scan whole table */
 	RangeVar   *relation;		/* single table to process, or NULL */
 	List	   *va_cols;		/* list of column names, or NIL for all */
+	/* place these at the end, to avoid ABI break within 9.3 branch */
+	int			multixact_freeze_min_age;		/* min multixact freeze age,
+												 * or -1 to use default */
+	int			multixact_freeze_table_age;		/* multixact age at which to
+												 * scan whole table */
 } VacuumStmt;
 
 /* ----------------------

src/include/postmaster/autovacuum.h

@@ -24,6 +24,7 @@ extern double autovacuum_vac_scale;
 extern int	autovacuum_anl_thresh;
 extern double autovacuum_anl_scale;
 extern int	autovacuum_freeze_max_age;
+extern int	autovacuum_multixact_freeze_max_age;
 extern int	autovacuum_vac_cost_delay;
 extern int	autovacuum_vac_cost_limit;
 

src/include/utils/rel.h

@@ -187,7 +187,11 @@ typedef struct RelationData
  * be applied to relations that use this format or a superset for
  * private options data.
  */
- /* autovacuum-related reloptions. */
+ /*
+  * autovacuum-related reloptions.
+  *
+  * Split in two to avoid ABI break.
+  */
 typedef struct AutoVacOpts
 {
 	bool		enabled;
@@ -202,12 +206,26 @@ typedef struct AutoVacOpts
 	float8		analyze_scale_factor;
 } AutoVacOpts;
 
+/*
+ * The multixact freeze parameters were added after 9.3.2 had been released;
+ * to preserve ABI compatibility with modules that might have been compiled
+ * prior to 9.3.3, these are placed in a separate struct so that they can be
+ * located at the end of the containing struct.
+ */
+typedef struct AutoVacOpts2
+{
+	int		multixact_freeze_min_age;
+	int		multixact_freeze_max_age;
+	int		multixact_freeze_table_age;
+} AutoVacOpts2;
+
 typedef struct StdRdOptions
 {
 	int32		vl_len_;		/* varlena header (do not touch directly!) */
 	int			fillfactor;		/* page fill factor in percent (0..100) */
 	AutoVacOpts autovacuum;		/* autovacuum-related options */
 	bool		security_barrier;		/* for views */
+	AutoVacOpts2 autovacuum2;	/* rest of autovacuum options */
 } StdRdOptions;
 
 #define HEAP_MIN_FILLFACTOR			10