remove apparently-unnecessary Result node traversal

add some comments

diff --git a/contrib/pg_plan_advice/pgpa_join.c b/contrib/pg_plan_advice/pgpa_join.c
index 2983ba8a2d58a3ff6d2438246eff2283a0ce8789..fbc4ad9d541dd351a0d4bfbc435bd2be4cbdcb13 100644 (file)
--- a/contrib/pg_plan_advice/pgpa_join.c
+++ b/contrib/pg_plan_advice/pgpa_join.c
@@ -380,12 +380,11 @@ pgpa_decompose_join(PlannedStmt *pstmt, Plan *plan,
        switch (nodeTag(plan))
        {
                case T_MergeJoin:
-                       if (elidedouter == NULL && is_sorting_plan(outerplan))
-                               elidedouter = pgpa_descend_node(pstmt, &outerplan);
-
-                       if (elidedouter == NULL && is_result_node_with_child(outerplan))
-                               elidedouter = pgpa_descend_node(pstmt, &outerplan);
-
+                       /*
+                        * The planner may have chosen to place a Material node on the
+                        * inner side of the MergeJoin; if this is present, we record it
+                        * as part of the join strategy.
+                        */
                        if (elidedinner == NULL && IsA(innerplan, Material))
                        {
                                elidedinner = pgpa_descend_node(pstmt, &innerplan);
@@ -394,14 +393,23 @@ pgpa_decompose_join(PlannedStmt *pstmt, Plan *plan,
                        else
                                strategy = JSTRAT_MERGE_JOIN_PLAIN;
 
+                       /*
+                        * For a MergeJoin, either the outer or the inner subplan, or
+                        * both, may have needed to be sorted; we must disregard any Sort
+                        * or IncrementalSort node to find the real inner or outer subplan.
+                        */
+                       if (elidedouter == NULL && is_sorting_plan(outerplan))
+                               elidedouter = pgpa_descend_node(pstmt, &outerplan);
                        if (elidedinner == NULL && is_sorting_plan(innerplan))
                                elidedinner = pgpa_descend_node(pstmt, &innerplan);
-
-                       if (elidedinner == NULL && is_result_node_with_child(innerplan))
-                               elidedinner = pgpa_descend_node(pstmt, &innerplan);
                        break;
 
                case T_NestLoop:
+                       /*
+                        * The planner may have chosen to place a Material or Memoize
+                        * node on the inner side of the NestLoop; if this is present,
+                        * we record it as part of the join strategy.
+                        */
                        if (elidedinner == NULL && IsA(innerplan, Material))
                        {
                                elidedinner = pgpa_descend_node(pstmt, &innerplan);
@@ -417,6 +425,10 @@ pgpa_decompose_join(PlannedStmt *pstmt, Plan *plan,
                        break;
 
                case T_HashJoin:
+                       /*
+                        * The inner subplan of a HashJoin is always a Hash node; the real
+                        * inner subplan is the Hash node's child.
+                        */
                        Assert(IsA(innerplan, Hash));
                        Assert(elidedinner == NULL);
                        elidedinner = pgpa_descend_node(pstmt, &innerplan);