Use SET TRANSACTION READ ONLY in pg_dump, if server supports it.

This currently does little except serve as documentation.  (The one case
where it has a performance benefit, SERIALIZABLE mode in 9.1 and up, was
already using READ ONLY mode.)  However, it's possible that it might have
performance benefits in future, and in any case it seems like good
practice since it would catch any accidentally non-read-only operations.

Pavan Deolasee

src/bin/pg_dump/pg_dump.c

@@ -618,7 +618,14 @@ main(int argc, char **argv)
 		else
 			ExecuteSqlStatement(fout,
 								"SET TRANSACTION ISOLATION LEVEL "
-								"REPEATABLE READ");
+								"REPEATABLE READ, READ ONLY");
+	}
+	else if (fout->remoteVersion >= 70400)
+	{
+		/* note: comma was not accepted in SET TRANSACTION before 8.0 */
+		ExecuteSqlStatement(fout,
+							"SET TRANSACTION ISOLATION LEVEL "
+							"SERIALIZABLE READ ONLY");
 	}
 	else
 		ExecuteSqlStatement(fout,