diff --git a/src/bin/scripts/reindexdb.c b/src/bin/scripts/reindexdb.c
index f93f7dd5fe..f00aec15de 100644
--- a/src/bin/scripts/reindexdb.c
+++ b/src/bin/scripts/reindexdb.c
@@ -153,12 +153,6 @@ main(int argc, char *argv[])
 					pg_log_error("number of parallel jobs must be at least 1");
 					exit(1);
 				}
-				if (concurrentCons > ParallelSlotsMax())
-				{
-					pg_log_error("too many parallel jobs requested (maximum: %d)",
-								 ParallelSlotsMax());
-					exit(1);
-				}
 				break;
 			case 'v':
 				verbose = true;
diff --git a/src/bin/scripts/scripts_parallel.c b/src/bin/scripts/scripts_parallel.c
index 10379a1f99..55bda9044b 100644
--- a/src/bin/scripts/scripts_parallel.c
+++ b/src/bin/scripts/scripts_parallel.c
@@ -94,20 +94,6 @@ select_loop(int maxFd, fd_set *workerset, bool *aborting)
 	return i;
 }
 
-/*
- * ParallelSlotsMax
- *		Returns the maximum number of parallel slots supported.
- *
- * Note that this is included here as FD_SETSIZE is declared in sys/select.h
- * per POSIX.
- */
-int
-ParallelSlotsMax(void)
-{
-	/* leave some room for pre-existing fds */
-	return FD_SETSIZE - 10;
-}
-
 /*
  * ParallelSlotsGetIdle
  *		Return a connection slot that is ready to execute a command.
@@ -246,6 +232,18 @@ ParallelSlotsSetup(const char *dbname, const char *host, const char *port,
 		{
 			conn = connectDatabase(dbname, host, port, username, prompt_password,
 								   progname, echo, false, true);
+
+			/*
+			 * Fail and exit immediately if trying to use a socket in an
+			 * unsupported range.  POSIX requires open(2) to use the lowest
+			 * unused file descriptor and the hint given relies on that.
+			 */
+			if (PQsocket(conn) >= FD_SETSIZE)
+			{
+				pg_log_fatal("too many jobs for this platform -- try %d", i);
+				exit(1);
+			}
+
 			init_slot(slots + i, conn);
 		}
 	}
diff --git a/src/bin/scripts/scripts_parallel.h b/src/bin/scripts/scripts_parallel.h
index 8042345072..ab82c5e6a9 100644
--- a/src/bin/scripts/scripts_parallel.h
+++ b/src/bin/scripts/scripts_parallel.h
@@ -21,8 +21,6 @@ typedef struct ParallelSlot
 	bool		isFree;			/* Is it known to be idle? */
 } ParallelSlot;
 
-extern int	ParallelSlotsMax(void);
-
 extern ParallelSlot *ParallelSlotsGetIdle(ParallelSlot *slots, int numslots);
 
 extern ParallelSlot *ParallelSlotsSetup(const char *dbname, const char *host,
diff --git a/src/bin/scripts/vacuumdb.c b/src/bin/scripts/vacuumdb.c
index c5c38692ed..2c7219239f 100644
--- a/src/bin/scripts/vacuumdb.c
+++ b/src/bin/scripts/vacuumdb.c
@@ -181,12 +181,6 @@ main(int argc, char *argv[])
 					pg_log_error("number of parallel jobs must be at least 1");
 					exit(1);
 				}
-				if (concurrentCons > ParallelSlotsMax())
-				{
-					pg_log_error("too many parallel jobs requested (maximum: %d)",
-								 ParallelSlotsMax());
-					exit(1);
-				}
 				break;
 			case 2:
 				maintenance_db = pg_strdup(optarg);