Allow executor startup pruning to prune all child nodes.

Previously, if the startup pruning logic proved that all child nodes
of an Append or MergeAppend could be pruned, we still kept one, just
to keep EXPLAIN from failing.  The previous commit removed the
ruleutils.c limitation that required this kluge, so drop it.  That
results in less-confusing EXPLAIN output, as per a complaint from
Yuzuko Hosoya.

David Rowley

Discussion: https://postgr.es/m/001001d4f44b$2a2cca50$7e865ef0$@lab.ntt.co.jp

1 files changed, 5 insertions, 25 deletions

diff --git a/src/backend/executor/nodeMergeAppend.c b/src/backend/executor/nodeMergeAppend.c
index 18d13377dc3..e6896eff742 100644
--- a/src/backend/executor/nodeMergeAppend.c
+++ b/src/backend/executor/nodeMergeAppend.c
@@ -80,7 +80,6 @@ ExecInitMergeAppend(MergeAppend *node, EState *estate, int eflags)
 	mergestate->ps.plan = (Plan *) node;
 	mergestate->ps.state = estate;
 	mergestate->ps.ExecProcNode = ExecMergeAppend;
-	mergestate->ms_noopscan = false;
 
 	/* If run-time partition pruning is enabled, then set that up now */
 	if (node->part_prune_info != NULL)
@@ -101,23 +100,6 @@ ExecInitMergeAppend(MergeAppend *node, EState *estate, int eflags)
 			validsubplans = ExecFindInitialMatchingSubPlans(prunestate,
 															list_length(node->mergeplans));
 
-			/*
-			 * The case where no subplans survive pruning must be handled
-			 * specially.  The problem here is that code in explain.c requires
-			 * a MergeAppend to have at least one subplan in order for it to
-			 * properly determine the Vars in that subplan's targetlist.  We
-			 * sidestep this issue by just initializing the first subplan and
-			 * setting ms_noopscan to true to indicate that we don't really
-			 * need to scan any subnodes.
-			 */
-			if (bms_is_empty(validsubplans))
-			{
-				mergestate->ms_noopscan = true;
-
-				/* Mark the first as valid so that it's initialized below */
-				validsubplans = bms_make_singleton(0);
-			}
-
 			nplans = bms_num_members(validsubplans);
 		}
 		else
@@ -129,14 +111,12 @@ ExecInitMergeAppend(MergeAppend *node, EState *estate, int eflags)
 		}
 
 		/*
-		 * If no runtime pruning is required, we can fill ms_valid_subplans
-		 * immediately, preventing later calls to ExecFindMatchingSubPlans.
+		 * When no run-time pruning is required and there's at least one
+		 * subplan, we can fill as_valid_subplans immediately, preventing
+		 * later calls to ExecFindMatchingSubPlans.
 		 */
-		if (!prunestate->do_exec_prune)
-		{
-			Assert(nplans > 0);
+		if (!prunestate->do_exec_prune && nplans > 0)
 			mergestate->ms_valid_subplans = bms_add_range(NULL, 0, nplans - 1);
-		}
 	}
 	else
 	{
@@ -240,7 +220,7 @@ ExecMergeAppend(PlanState *pstate)
 	if (!node->ms_initialized)
 	{
 		/* Nothing to do if all subplans were pruned */
-		if (node->ms_noopscan)
+		if (node->ms_nplans == 0)
 			return ExecClearTuple(node->ps.ps_ResultTupleSlot);
 
 		/*