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.
+				 */
 			}
 		}
 	}