vacuumlazy.c: MultiXactIds are MXIDs, not XMIDs.

Oversights in commits 0b018fa and f3c15cb.

Author: petergeoghegan

src/backend/access/heap/vacuumlazy.c

@@ -512,7 +512,7 @@ heap_vacuum_rel(Relation rel, VacuumParams *params,
 vacrel->FreezeLimit = FreezeLimit;
 /* MultiXactCutoff controls MXID freezing (always <= OldestMxact) */
 vacrel->MultiXactCutoff = MultiXactCutoff;
-/* Initialize state used to track oldest extant XID/XMID */
+/* Initialize state used to track oldest extant XID/MXID */
 vacrel->NewRelfrozenXid = OldestXmin;
 vacrel->NewRelminMxid = OldestMxact;
 vacrel->skippedallvis = false;
@@ -1295,7 +1295,7 @@ lazy_scan_heap(LVRelState *vacrel)
  * Note: our opinion of which blocks can be skipped can go stale immediately.
  * It's okay if caller "misses" a page whose all-visible or all-frozen marking
  * was concurrently cleared, though. All that matters is that caller scan all
- * pages whose tuples might contain XIDs < OldestXmin, or XMIDs < OldestMxact.
+ * pages whose tuples might contain XIDs < OldestXmin, or MXIDs < OldestMxact.
  * (Actually, non-aggressive VACUUMs can choose to skip all-visible pages with
  * older XIDs/MXIDs. The vacrel->skippedallvis flag will be set here when the
  * choice to skip such a range is actually made, making everything safe.)
@@ -2012,7 +2012,7 @@ lazy_scan_noprune(LVRelState *vacrel,
  * relfrozenxid to a value >= FreezeLimit (and be able to
  * advance rel's relminmxid to a value >= MultiXactCutoff).
  * The ongoing aggressive VACUUM won't be able to do that
- * unless it can freeze an XID (or XMID) from this tuple now.
+ * unless it can freeze an XID (or MXID) from this tuple now.
  *
  * The only safe option is to have caller perform processing
  * of this page using lazy_scan_prune. Caller might have to