Fix the size of predicate lock manager's shared memory hash tables at creation.

This way they don't compete with the regular lock manager for the slack shared
memory, making the behavior more predictable.

src/backend/utils/hash/dynahash.c

@@ -160,6 +160,7 @@ struct HTAB
 	MemoryContext hcxt;			/* memory context if default allocator used */
 	char	   *tabname;		/* table name (for error messages) */
 	bool		isshared;		/* true if table is in shared memory */
+	bool		isfixed;		/* if true, don't enlarge */
 
 	/* freezing a shared table isn't allowed, so we can keep state here */
 	bool		frozen;			/* true = no more inserts allowed */
@@ -435,6 +436,8 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
 					 errmsg("out of memory")));
 	}
 
+	if (flags & HASH_FIXED_SIZE)
+		hashp->isfixed = true;
 	return hashp;
 }
 
@@ -1334,6 +1337,9 @@ element_alloc(HTAB *hashp, int nelem)
 	HASHELEMENT *prevElement;
 	int			i;
 
+	if (hashp->isfixed)
+		return false;
+
 	/* Each element has a HASHELEMENT header plus user data. */
 	elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(hctlv->entrysize);