diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index dce355e6683..7a9d5c6198b 100644
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -125,11 +125,14 @@ void
 InitShmemAllocator(PGShmemHeader *seghdr)
 {
 	Size		offset;
-	int64		hash_size;
 	HASHCTL		info;
 	int			hash_flags;
 	size_t		size;
 
+#ifndef EXEC_BACKEND
+	Assert(!IsUnderPostmaster);
+#endif
+
 	Assert(seghdr != NULL);
 
 	/*
@@ -139,10 +142,6 @@ InitShmemAllocator(PGShmemHeader *seghdr)
 	Assert(seghdr == (void *) MAXALIGN(seghdr));
 	Assert(seghdr->content_offset == MAXALIGN(seghdr->content_offset));
 
-	ShmemSegHdr = seghdr;
-	ShmemBase = seghdr;
-	ShmemEnd = (char *) ShmemBase + seghdr->totalsize;
-
 	/*
 	 * Allocations after this point should go through ShmemAlloc, which
 	 * expects to allocate everything on cache line boundaries.  Make sure the
@@ -155,18 +154,25 @@ InitShmemAllocator(PGShmemHeader *seghdr)
 				 errmsg("out of shared memory (%zu bytes requested)",
 						offset)));
 
+	/*
+	 * In postmaster or stand-alone backend, initialize the shared memory
+	 * allocator and the spinlock so that we can allocate shared memory for
+	 * ShmemIndex using ShmemAlloc(). In a regular backend just set up the
+	 * pointers required by ShmemAlloc().
+	 */
 	ShmemAllocator = (ShmemAllocatorData *) ((char *) seghdr + seghdr->content_offset);
-	ShmemLock = &ShmemAllocator->shmem_lock;
 
-#ifndef EXEC_BACKEND
-	Assert(!IsUnderPostmaster);
-#endif
 	if (!IsUnderPostmaster)
 	{
 		SpinLockInit(&ShmemAllocator->shmem_lock);
 		ShmemAllocator->free_offset = offset;
 	}
 
+	ShmemLock = &ShmemAllocator->shmem_lock;
+	ShmemSegHdr = seghdr;
+	ShmemBase = seghdr;
+	ShmemEnd = (char *) ShmemBase + seghdr->totalsize;
+
 	/*
 	 * Create (or attach to) the shared memory index of shmem areas.
 	 *
@@ -174,11 +180,9 @@ InitShmemAllocator(PGShmemHeader *seghdr)
 	 * use ShmemInitHash() here because it relies on ShmemIndex being already
 	 * initialized.
 	 */
-	hash_size = SHMEM_INDEX_SIZE;
-
 	info.keysize = SHMEM_INDEX_KEYSIZE;
 	info.entrysize = sizeof(ShmemIndexEnt);
-	info.dsize = info.max_dsize = hash_select_dirsize(hash_size);
+	info.dsize = info.max_dsize = hash_select_dirsize(SHMEM_INDEX_SIZE);
 	info.alloc = ShmemAllocNoError;
 	hash_flags = HASH_ELEM | HASH_STRINGS | HASH_SHARED_MEM | HASH_ALLOC | HASH_DIRSIZE;
 	if (!IsUnderPostmaster)
@@ -189,7 +193,7 @@ InitShmemAllocator(PGShmemHeader *seghdr)
 	else
 		hash_flags |= HASH_ATTACH;
 	info.hctl = ShmemAllocator->index;
-	ShmemIndex = hash_create("ShmemIndex", hash_size, &info, hash_flags);
+	ShmemIndex = hash_create("ShmemIndex", SHMEM_INDEX_SIZE, &info, hash_flags);
 	Assert(ShmemIndex != NULL);
 }