3 files changed, 39 insertions, 18 deletions

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 7ed03b33a7..8fbf110b13 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -3374,7 +3374,11 @@ make_rel_from_joinlist(PlannerInfo *root, List *joinlist)
 		if (enable_hashjoin)
 			jsa_mask |= JSA_HASHJOIN;
 		if (enable_mergejoin)
-			jsa_mask |= JSA_MERGEJOIN;
+		{
+			jsa_mask |= JSA_MERGEJOIN_PLAIN;
+			if (enable_material)
+				jsa_mask |= JSA_MERGEJOIN_MATERIALIZE;
+		}
 		if (enable_nestloop)
 		{
 			jsa_mask |= JSA_NESTLOOP_PLAIN;
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 38ad8fb50e..0700c634bf 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -3676,7 +3676,12 @@ initial_cost_mergejoin(PlannerInfo *root, JoinCostWorkspace *workspace,
 	Assert(outerstartsel <= outerendsel);
 	Assert(innerstartsel <= innerendsel);
 
-	disabled_nodes = (extra->jsa_mask & JSA_MERGEJOIN) == 0 ? 1 : 0;
+	/*
+	 * Assume for now that this node is not itself disabled. We'll sort out
+	 * whether that's really the case in final_cost_mergejoin(); here, we'll
+	 * just account for any disabled child nodes.
+	 */
+	disabled_nodes = 0;
 
 	/* cost of source data */
 
@@ -3814,9 +3819,6 @@ final_cost_mergejoin(PlannerInfo *root, MergePath *path,
 				rescannedtuples;
 	double		rescanratio;
 
-	/* Set the number of disabled nodes. */
-	path->jpath.path.disabled_nodes = workspace->disabled_nodes;
-
 	/* Protect some assumptions below that rowcounts aren't zero */
 	if (inner_path_rows <= 0)
 		inner_path_rows = 1;
@@ -3943,16 +3945,20 @@ final_cost_mergejoin(PlannerInfo *root, MergePath *path,
 		path->materialize_inner = false;
 
 	/*
-	 * Prefer materializing if it looks cheaper, unless the user has asked to
-	 * suppress materialization.
+	 * If merge joins with materialization are enabled, then choose
+	 * materialization if either (a) it looks cheaper or (b) merge joins
+	 * without materialization are disabled.
 	 */
-	else if (enable_material && mat_inner_cost < bare_inner_cost)
+	else if ((extra->jsa_mask & JSA_MERGEJOIN_MATERIALIZE) != 0 &&
+			 (mat_inner_cost < bare_inner_cost ||
+			  (extra->jsa_mask & JSA_MERGEJOIN_PLAIN) == 0))
 		path->materialize_inner = true;
 
 	/*
-	 * Even if materializing doesn't look cheaper, we *must* do it if the
-	 * inner path is to be used directly (without sorting) and it doesn't
-	 * support mark/restore.
+	 * Regardless of what plan shapes are enabled and what the costs seem
+	 * to be, we *must* materialize it if the inner path is to be used directly
+	 * (without sorting) and it doesn't support mark/restore. Planner failure
+	 * is not an option!
 	 *
 	 * Since the inner side must be ordered, and only Sorts and IndexScans can
 	 * create order to begin with, and they both support mark/restore, you
@@ -3960,10 +3966,6 @@ final_cost_mergejoin(PlannerInfo *root, MergePath *path,
 	 * merge joins can *preserve* the order of their inputs, so they can be
 	 * selected as the input of a mergejoin, and they don't support
 	 * mark/restore at present.
-	 *
-	 * We don't test the value of enable_material here, because
-	 * materialization is required for correctness in this case, and turning
-	 * it off does not entitle us to deliver an invalid plan.
 	 */
 	else if (innersortkeys == NIL &&
 			 !ExecSupportsMarkRestore(inner_path))
@@ -3980,7 +3982,8 @@ final_cost_mergejoin(PlannerInfo *root, MergePath *path,
 	 * rather than necessary for correctness, we skip it if enable_material is
 	 * off.
 	 */
-	else if (enable_material && innersortkeys != NIL &&
+	else if ((extra->jsa_mask & JSA_MERGEJOIN_MATERIALIZE) != 0 &&
+			 innersortkeys != NIL &&
 			 relation_byte_size(inner_path_rows,
 								inner_path->pathtarget->width) >
 			 (work_mem * 1024L))
@@ -3988,11 +3991,25 @@ final_cost_mergejoin(PlannerInfo *root, MergePath *path,
 	else
 		path->materialize_inner = false;
 
-	/* Charge the right incremental cost for the chosen case */
+	/* Get the number of disabled nodes, not yet including this one. */
+	path->jpath.path.disabled_nodes = workspace->disabled_nodes;
+
+	/*
+	 * Charge the right incremental cost for the chosen case, and increment
+	 * disabled_nodes if appropriate.
+	 */
 	if (path->materialize_inner)
+	{
 		run_cost += mat_inner_cost;
+		if ((extra->jsa_mask & JSA_MERGEJOIN_MATERIALIZE) == 0)
+			++path->jpath.path.disabled_nodes;
+	}
 	else
+	{
 		run_cost += bare_inner_cost;
+		if ((extra->jsa_mask & JSA_MERGEJOIN_PLAIN) == 0)
+			++path->jpath.path.disabled_nodes;
+	}
 
 	/* CPU costs */
 
diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
index d853e2a64e..0ef7e776fe 100644
--- a/src/backend/optimizer/path/joinpath.c
+++ b/src/backend/optimizer/path/joinpath.c
@@ -237,7 +237,7 @@ add_paths_to_joinrel(PlannerInfo *root,
 	 * way of implementing a full outer join, so in that case we don't care
 	 * whether mergejoins are disabled.
 	 */
-	if ((extra.jsa_mask & JSA_MERGEJOIN) != 0 || jointype == JOIN_FULL)
+	if ((extra.jsa_mask & JSA_MERGEJOIN_ANY) != 0 || jointype == JOIN_FULL)
 		extra.mergeclause_list = select_mergejoin_clauses(root,
 														  joinrel,
 														  outerrel,