From: Robert Haas <rhaas@postgresql.org>
Date: Tue, 24 Jun 2025 16:22:16 +0000 (-0400)
Subject: more comments
X-Git-Url: http://git.postgresql.org/gitweb/static/gitweb.js?a=commitdiff_plain;h=fd172250585162aa569648b44eadfac6775a2df2;p=users%2Frhaas%2Fpostgres.git

more comments
---

diff --git a/contrib/pg_plan_advice/pgpa_join.c b/contrib/pg_plan_advice/pgpa_join.c
index 1bb8e2c91b..81fec87169 100644
--- 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);