Fix executor prune failure when plan already pruned

In a multi-layer partitioning setup, if at plan time all the
sub-partitions are pruned but the intermediate one remains, the executor
later throws a spurious error that there's nothing to prune.  That is
correct, but there's no reason to throw an error.  Therefore, don't.
Reported-by: Andreas Seltenreich <seltenreich@gmx.de>
Author: David Rowley <david.rowley@2ndquadrant.com>
Discussion: https://postgr.es/m/87in4h98i0.fsf@ansel.ydns.eu

src/backend/executor/execPartition.c

@@ -1886,8 +1886,13 @@ find_matching_subplans_recurse(PartitionPruningData *prunedata,
 											   initial_prune, validsubplans);
 			else
 			{
-				/* Shouldn't happen */
-				elog(ERROR, "partition missing from subplans");
+				/*
+				 * We get here if the planner already pruned all the sub-
+				 * partitions for this partition.  Silently ignore this
+				 * partition in this case.  The end result is the same: we
+				 * would have pruned all partitions just the same, but we
+				 * don't have any pruning steps to execute to verify this.
+				 */
 			}
 		}
 	}

src/test/regress/expected/partition_prune.out

@@ -3570,3 +3570,21 @@ execute q (1, 1);
 
 reset plan_cache_mode;
 drop table p, q;
+-- Ensure run-time pruning works correctly when we match a partitioned table
+-- on the first level but find no matching partitions on the second level.
+create table listp (a int, b int) partition by list (a);
+create table listp1 partition of listp for values in(1);
+create table listp2 partition of listp for values in(2) partition by list(b);
+create table listp2_10 partition of listp2 for values in (10);
+explain (analyze, costs off, summary off, timing off)
+select * from listp where a = (select 2) and b <> 10;
+                QUERY PLAN                 
+-------------------------------------------
+ Append (actual rows=0 loops=1)
+   InitPlan 1 (returns $0)
+     ->  Result (actual rows=1 loops=1)
+   ->  Seq Scan on listp1 (never executed)
+         Filter: ((b <> 10) AND (a = $0))
+(5 rows)
+
+drop table listp;

src/test/regress/sql/partition_prune.sql

@@ -946,3 +946,15 @@ execute q (1, 1);
 
 reset plan_cache_mode;
 drop table p, q;
+
+-- Ensure run-time pruning works correctly when we match a partitioned table
+-- on the first level but find no matching partitions on the second level.
+create table listp (a int, b int) partition by list (a);
+create table listp1 partition of listp for values in(1);
+create table listp2 partition of listp for values in(2) partition by list(b);
+create table listp2_10 partition of listp2 for values in (10);
+
+explain (analyze, costs off, summary off, timing off)
+select * from listp where a = (select 2) and b <> 10;
+
+drop table listp;