Actually, all of parallel restore's limitations should be tested earlier.

On closer inspection, whining in restore_toc_entries_parallel is really
much too late for any user-facing error case.  The right place to do it
is at the start of RestoreArchive(), before we've done anything interesting
(suh as trying to DROP all the targets ...)

Back-patch to 8.4, where parallel restore was introduced.

src/bin/pg_dump/pg_backup_archiver.c

@@ -213,6 +213,7 @@ void
 RestoreArchive(Archive *AHX, RestoreOptions *ropt)
 {
 	ArchiveHandle *AH = (ArchiveHandle *) AHX;
+	bool		parallel_mode;
 	TocEntry   *te;
 	teReqs		reqs;
 	OutputContext sav;
@@ -238,6 +239,27 @@ RestoreArchive(Archive *AHX, RestoreOptions *ropt)
 	if (ropt->createDB && ropt->single_txn)
 		die_horribly(AH, modulename, "-C and -1 are incompatible options\n");
 
+	/*
+	 * If we're going to do parallel restore, there are some restrictions.
+	 */
+	parallel_mode = (ropt->number_of_jobs > 1 && ropt->useDB);
+	if (parallel_mode)
+	{
+		/* We haven't got round to making this work for all archive formats */
+		if (AH->ClonePtr == NULL || AH->ReopenPtr == NULL)
+			die_horribly(AH, modulename, "parallel restore is not supported with this archive file format\n");
+
+		/* Doesn't work if the archive represents dependencies as OIDs */
+		if (AH->version < K_VERS_1_8)
+			die_horribly(AH, modulename, "parallel restore is not supported with archives made by pre-8.0 pg_dump\n");
+
+		/*
+		 * It's also not gonna work if we can't reopen the input file, so
+		 * let's try that immediately.
+		 */
+		(AH->ReopenPtr) (AH);
+	}
+
 	/*
 	 * Make sure we won't need (de)compression we haven't got
 	 */
@@ -385,7 +407,7 @@ RestoreArchive(Archive *AHX, RestoreOptions *ropt)
 	 *
 	 * In parallel mode, turn control over to the parallel-restore logic.
 	 */
-	if (ropt->number_of_jobs > 1 && ropt->useDB)
+	if (parallel_mode)
 		restore_toc_entries_parallel(AH);
 	else
 	{
@@ -3257,20 +3279,6 @@ restore_toc_entries_parallel(ArchiveHandle *AH)
 
 	ahlog(AH, 2, "entering restore_toc_entries_parallel\n");
 
-	/* we haven't got round to making this work for all archive formats */
-	if (AH->ClonePtr == NULL || AH->ReopenPtr == NULL)
-		die_horribly(AH, modulename, "parallel restore is not supported with this archive file format\n");
-
-	/* doesn't work if the archive represents dependencies as OIDs, either */
-	if (AH->version < K_VERS_1_8)
-		die_horribly(AH, modulename, "parallel restore is not supported with archives made by pre-8.0 pg_dump\n");
-
-	/*
-	 * It's also not gonna work if we can't reopen the input file, so let's
-	 * try that immediately.
-	 */
-	(AH->ReopenPtr) (AH);
-
 	slots = (ParallelSlot *) calloc(sizeof(ParallelSlot), n_slots);
 
 	/* Adjust dependency information */