3 files changed, 26 insertions, 12 deletions

diff --git a/src/backend/access/nbtree/nbtutils.c b/src/backend/access/nbtree/nbtutils.c
index 6a651d8397..84164748b3 100644
--- a/src/backend/access/nbtree/nbtutils.c
+++ b/src/backend/access/nbtree/nbtutils.c
@@ -2072,7 +2072,7 @@ BTreeShmemSize(void)
 	Size		size;
 
 	size = offsetof(BTVacInfo, vacuums);
-	size = add_size(size, mul_size(MaxBackends, sizeof(BTOneVacInfo)));
+	size = add_size(size, mul_size(GetMaxBackends(), sizeof(BTOneVacInfo)));
 	return size;
 }
 
@@ -2101,7 +2101,7 @@ BTreeShmemInit(void)
 		btvacinfo->cycle_ctr = (BTCycleId) time(NULL);
 
 		btvacinfo->num_vacuums = 0;
-		btvacinfo->max_vacuums = MaxBackends;
+		btvacinfo->max_vacuums = GetMaxBackends();
 	}
 	else
 		Assert(found);
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 806f2e43ba..f00c272521 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -282,12 +282,11 @@ typedef struct MultiXactStateData
 } MultiXactStateData;
 
 /*
- * Last element of OldestMemberMXactId and OldestVisibleMXactId arrays.
- * Valid elements are (1..MaxOldestSlot); element 0 is never used.
+ * Pointers to the state data in shared memory
+ *
+ * The index of the last element of the OldestMemberMXactId and
+ * OldestVisibleMXacId arrays can be obtained with GetMaxOldestSlot().
  */
-#define MaxOldestSlot	(MaxBackends + max_prepared_xacts)
-
-/* Pointers to the state data in shared memory */
 static MultiXactStateData *MultiXactState;
 static MultiXactId *OldestMemberMXactId;
 static MultiXactId *OldestVisibleMXactId;
@@ -342,6 +341,7 @@ static void MultiXactIdSetOldestVisible(void);
 static void RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
 							   int nmembers, MultiXactMember *members);
 static MultiXactId GetNewMultiXactId(int nmembers, MultiXactOffset *offset);
+static inline int GetMaxOldestSlot(void);
 
 /* MultiXact cache management */
 static int	mxactMemberComparator(const void *arg1, const void *arg2);
@@ -663,6 +663,17 @@ MultiXactIdSetOldestMember(void)
 }
 
 /*
+ * Retrieve the index of the last element of the OldestMemberMXactId and
+ * OldestVisibleMXactId arrays.  Valid elements are (1..MaxOldestSlot); element
+ * 0 is never used.
+ */
+static inline int
+GetMaxOldestSlot(void)
+{
+	return GetMaxBackends() + max_prepared_xacts;
+}
+
+/*
  * MultiXactIdSetOldestVisible
  *		Save the oldest MultiXactId this transaction considers possibly live.
  *
@@ -684,6 +695,7 @@ MultiXactIdSetOldestVisible(void)
 	if (!MultiXactIdIsValid(OldestVisibleMXactId[MyBackendId]))
 	{
 		MultiXactId oldestMXact;
+		int			maxOldestSlot = GetMaxOldestSlot();
 		int			i;
 
 		LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
@@ -697,7 +709,7 @@ MultiXactIdSetOldestVisible(void)
 		if (oldestMXact < FirstMultiXactId)
 			oldestMXact = FirstMultiXactId;
 
-		for (i = 1; i <= MaxOldestSlot; i++)
+		for (i = 1; i <= maxOldestSlot; i++)
 		{
 			MultiXactId thisoldest = OldestMemberMXactId[i];
 
@@ -1831,7 +1843,7 @@ MultiXactShmemSize(void)
 	/* We need 2*MaxOldestSlot + 1 perBackendXactIds[] entries */
 #define SHARED_MULTIXACT_STATE_SIZE \
 	add_size(offsetof(MultiXactStateData, perBackendXactIds) + sizeof(MultiXactId), \
-			 mul_size(sizeof(MultiXactId) * 2, MaxOldestSlot))
+			 mul_size(sizeof(MultiXactId) * 2, GetMaxOldestSlot()))
 
 	size = SHARED_MULTIXACT_STATE_SIZE;
 	size = add_size(size, SimpleLruShmemSize(NUM_MULTIXACTOFFSET_BUFFERS, 0));
@@ -1882,7 +1894,7 @@ MultiXactShmemInit(void)
 	 * since we only use indexes 1..MaxOldestSlot in each array.
 	 */
 	OldestMemberMXactId = MultiXactState->perBackendXactIds;
-	OldestVisibleMXactId = OldestMemberMXactId + MaxOldestSlot;
+	OldestVisibleMXactId = OldestMemberMXactId + GetMaxOldestSlot();
 }
 
 /*
@@ -2507,6 +2519,7 @@ GetOldestMultiXactId(void)
 {
 	MultiXactId oldestMXact;
 	MultiXactId nextMXact;
+	int			maxOldestSlot = GetMaxOldestSlot();
 	int			i;
 
 	/*
@@ -2525,7 +2538,7 @@ GetOldestMultiXactId(void)
 		nextMXact = FirstMultiXactId;
 
 	oldestMXact = nextMXact;
-	for (i = 1; i <= MaxOldestSlot; i++)
+	for (i = 1; i <= maxOldestSlot; i++)
 	{
 		MultiXactId thisoldest;
 
diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index 271a3146db..608c5149e5 100644
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -260,6 +260,7 @@ TwoPhaseShmemInit(void)
 	{
 		GlobalTransaction gxacts;
 		int			i;
+		int			max_backends = GetMaxBackends();
 
 		Assert(!found);
 		TwoPhaseState->freeGXacts = NULL;
@@ -293,7 +294,7 @@ TwoPhaseShmemInit(void)
 			 * prepared transaction. Currently multixact.c uses that
 			 * technique.
 			 */
-			gxacts[i].dummyBackendId = MaxBackends + 1 + i;
+			gxacts[i].dummyBackendId = max_backends + 1 + i;
 		}
 	}
 	else