Don't scan partitioned tables.

Partitioned tables do not contain any data; only their unpartitioned
descendents need to be scanned.  However, the partitioned tables still
need to be locked, even though they're not scanned.  To make that
work, Append and MergeAppend relations now need to carry a list of
(unscanned) partitioned relations that must be locked, and InitPlan
must lock all partitioned result relations.

Aside from the obvious advantage of avoiding some work at execution
time, this has two other advantages.  First, it may improve the
planner's decision-making in some cases since the empty relation
might throw things off.  Second, it paves the way to getting rid of
the storage for partitioned tables altogether.

Amit Langote, reviewed by me.

Discussion: http://postgr.es/m/6837c359-45c4-8044-34d1-736756335a15@lab.ntt.co.jp

src/backend/executor/nodeMergeAppend.c

@@ -71,6 +71,12 @@ ExecInitMergeAppend(MergeAppend *node, EState *estate, int eflags)
 	/* check for unsupported flags */
 	Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));
 
+	/*
+	 * Lock the non-leaf tables in the partition tree controlled by this
+	 * node.  It's a no-op for non-partitioned parent tables.
+	 */
+	ExecLockNonLeafAppendTables(node->partitioned_rels, estate);
+
 	/*
 	 * Set up empty vector of subplan states
 	 */