Fix full-table-vacuum request mechanism for MultiXactIds

While autovacuum dutifully launched anti-multixact-wraparound vacuums
when the multixact "age" was reached, the vacuum code was not aware that
it needed to make them be full table vacuums.  As the resulting
partial-table vacuums aren't capable of actually increasing relminmxid,
autovacuum continued to launch anti-wraparound vacuums that didn't have
the intended effect, until age of relfrozenxid caused the vacuum to
finally be a full table one via vacuum_freeze_table_age.

To fix, introduce logic for multixacts similar to that for plain
TransactionIds, using the same GUCs.

Backpatch to 9.3, where permanent MultiXactIds were introduced.

Andres Freund, some cleanup by Álvaro

1 files changed, 15 insertions, 0 deletions

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index de0193aaf64..90fa030caf2 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -2375,6 +2375,21 @@ MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2)
 }
 
 /*
+ * MultiXactIdPrecedesOrEquals -- is multi1 logically <= multi2?
+ *
+ * XXX do we need to do something special for InvalidMultiXactId?
+ * (Doesn't look like it.)
+ */
+bool
+MultiXactIdPrecedesOrEquals(MultiXactId multi1, MultiXactId multi2)
+{
+	int32		diff = (int32) (multi1 - multi2);
+
+	return (diff <= 0);
+}
+
+
+/*
  * Decide which of two offsets is earlier.
  */
 static bool