more comments

diff --git a/contrib/pg_plan_advice/pgpa_join.c b/contrib/pg_plan_advice/pgpa_join.c
index 1bb8e2c91bcd86f18eb6427ff6fa0ac828bae9de..81fec871699be857930a7544fd7fe92b23c01de6 100644 (file)
--- a/contrib/pg_plan_advice/pgpa_join.c
+++ b/contrib/pg_plan_advice/pgpa_join.c
@@ -444,21 +444,46 @@ pgpa_decompose_join(PlannedStmt *pstmt, Plan *plan,
                        elog(ERROR, "unrecognized node type: %d", (int) nodeTag(plan));
        }
 
+       /*
+        * The planner may have decided to implement a semijoin by first making
+        * the nullable side of the plan unique, and then performing a normal join
+        * against the result. Therefore, we might need to descend through a
+        * unique node on either side of the plan.
+        *
+        * XXX. We should really note the unique-ified rels here and emit advice.
+        */
        if (elidedouter == NULL)
                elidedouter = pgpa_descend_any_unique(pstmt, &outerplan);
-
        if (elidedinner == NULL)
                elidedinner = pgpa_descend_any_unique(pstmt, &innerplan);
 
+       /*
+        * The planner may have decided to parallelize part of the join tree, so
+        * we could find a Gather or Gather Merge node here. Note that, if
+        * present, this will appear below both (1) nodes we considered as part of
+        * the join strategy and (2) nodes inserted for uniqueness, because we
+        * currently only consider gathering a partial path on top of the final
+        * path for some RelOptInfo.
+        *
+        * XXX. We should really note the gathered rels here and emit advice.
+        */
        if (elidedouter == NULL)
                elidedouter = pgpa_descend_any_gather(pstmt, &outerplan);
-
        if (elidedinner == NULL)
                elidedinner = pgpa_descend_any_gather(pstmt, &innerplan);
 
+       /*
+        * It's possible that Result node has been inserted either to project a
+        * target list or to implement a one-time filter. If so, we can descend
+        * throught it. Note that a result node without a child would be a
+        * degenerate scan or join, and not something we could descend through.
+        *
+        * XXX. I suspect it's possible for this to happen above the Gather or
+        * Gather Merge node, too, but apparently we have no test case for that
+        * scenario.
+        */
        if (elidedouter == NULL && is_result_node_with_child(outerplan))
                elidedouter = pgpa_descend_node(pstmt, &outerplan);
-
        if (elidedinner == NULL && is_result_node_with_child(innerplan))
                elidedinner = pgpa_descend_node(pstmt, &innerplan);