*** a/src/backend/storage/ipc/shmem.c
--- b/src/backend/storage/ipc/shmem.c
***************
*** 268,273 **** ShmemInitHash(const char *name, /* table string name for shmem index */
--- 268,274 ----
  	 *
  	 * The shared memory allocator must be specified too.
  	 */
+ 	infoP->max_size = max_size;
  	infoP->dsize = infoP->max_dsize = hash_select_dirsize(max_size);
  	infoP->alloc = ShmemAlloc;
  	hash_flags |= HASH_SHARED_MEM | HASH_ALLOC | HASH_DIRSIZE;
*** a/src/backend/utils/hash/dynahash.c
--- b/src/backend/utils/hash/dynahash.c
***************
*** 129,134 **** struct HASHHDR
--- 129,135 ----
  	long		ffactor;		/* target fill factor */
  	long		max_dsize;		/* 'dsize' limit if directory is fixed size */
  	long		ssize;			/* segment size --- must be power of 2 */
+ 	long		max_size;		/* maximum number of entries expected */
  	int			sshift;			/* segment shift = log2(ssize) */
  	int			nelem_alloc;	/* number of entries to allocate at once */
  
***************
*** 368,373 **** hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
--- 369,375 ----
  	hdefault(hashp);
  
  	hctl = hashp->hctl;
+ 	hctl->max_size = info->max_size;
  
  	if (flags & HASH_PARTITION)
  	{
***************
*** 1333,1338 **** element_alloc(HTAB *hashp, int nelem)
--- 1335,1341 ----
  	HASHELEMENT *tmpElement;
  	HASHELEMENT *prevElement;
  	int			i;
+ 	bool		warningNeeded;
  
  	/* Each element has a HASHELEMENT header plus user data. */
  	elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(hctlv->entrysize);
***************
*** 1360,1369 **** element_alloc(HTAB *hashp, int nelem)
--- 1363,1378 ----
  	/* freelist could be nonempty if two backends did this concurrently */
  	firstElement->link = hctlv->freeList;
  	hctlv->freeList = prevElement;
+ 	warningNeeded = (hctlv->max_size > 0 && hctlv->nentries == hctlv->max_size);
  
  	if (IS_PARTITIONED(hctlv))
  		SpinLockRelease(&hctlv->mutex);
  
+ 	if (warningNeeded)
+ 		ereport(WARNING,
+ 				(errmsg("hash table \"%s\" has more entries than expected", hashp->tabname),
+ 				 errdetail("The maximum was set to %li on creation.", hctlv->max_size)));
+ 
  	return true;
  }
  
*** a/src/include/utils/hsearch.h
--- b/src/include/utils/hsearch.h
***************
*** 69,74 **** typedef struct HASHCTL
--- 69,75 ----
  	long		dsize;			/* (initial) directory size */
  	long		max_dsize;		/* limit to dsize if dir size is limited */
  	long		ffactor;		/* fill factor */
+ 	long		max_size;		/* maximum number of entries expected */
  	Size		keysize;		/* hash key length in bytes */
  	Size		entrysize;		/* total user element size in bytes */
  	HashValueFunc hash;			/* hash function */