Avoid failure to open dropped detached partition

When a partition is detached and immediately dropped, a prepared
statement could try to compute a new partition descriptor that includes
it.  This leads to this kind of error:
ERROR:  could not open relation with OID 457639

Avoid this by skipping the partition in expand_partitioned_rtentry if it
doesn't exist.

Noted by me while investigating bug #18559.  Kuntal Gosh helped to
identify the exact failure.

Backpatch to 14, where DETACH CONCURRENTLY was introduced.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Discussion: https://postgr.es/m/202408122233.bo4adt3vh5bi@alvherre.pgsql

Author: alvherre

src/backend/optimizer/util/inherit.c

@@ -386,8 +386,17 @@ expand_partitioned_rtentry(PlannerInfo *root, RelOptInfo *relinfo,
 		Index		childRTindex;
 		RelOptInfo *childrelinfo;
 
-		/* Open rel, acquiring required locks */
-		childrel = table_open(childOID, lockmode);
+		/*
+		 * Open rel, acquiring required locks.  If a partition was recently
+		 * detached and subsequently dropped, then opening it will fail.  In
+		 * this case, behave as though the partition had been pruned.
+		 */
+		childrel = try_table_open(childOID, lockmode);
+		if (childrel == NULL)
+		{
+			relinfo->live_parts = bms_del_member(relinfo->live_parts, i);
+			continue;
+		}
 
 		/*
 		 * Temporary partitions belonging to other sessions should have been