From: Robert Haas <rhaas@postgresql.org>
Date: Tue, 15 Jul 2025 17:13:53 +0000 (-0400)
Subject: PGPAQF_SEMIJOIN_(NON_)UNIQUE
X-Git-Url: http://git.postgresql.org/gitweb/static/gitweb.js?a=commitdiff_plain;h=ba9f3debdbabbd02dab74de2df88af167b017bb6;p=users%2Frhaas%2Fpostgres.git

PGPAQF_SEMIJOIN_(NON_)UNIQUE
---

diff --git a/contrib/pg_plan_advice/pgpa_join.c b/contrib/pg_plan_advice/pgpa_join.c
index d7a5c99ffd..14c0a444da 100644
--- a/contrib/pg_plan_advice/pgpa_join.c
+++ b/contrib/pg_plan_advice/pgpa_join.c
@@ -402,6 +402,7 @@ pgpa_decompose_join(pgpa_plan_walker_context *context, Plan *plan,
 					ElidedNode **elidedrealouter, ElidedNode **elidedrealinner)
 {
 	PlannedStmt *pstmt = context->pstmt;
+	JoinType	jointype = ((Join *) plan)->jointype;
 	Plan	   *outerplan = plan->lefttree;
 	Plan	   *innerplan = plan->righttree;
 	ElidedNode *elidedouter;
@@ -517,16 +518,31 @@ pgpa_decompose_join(pgpa_plan_walker_context *context, Plan *plan,
 		elidedinner = pgpa_descend_node(pstmt, &innerplan);
 
 	/*
-	 * If we descended through a unique node, make a note that the identified
-	 * inner or outer subplan should be treated as a query plan feature when
-	 * the main tree traversal reaches it. (We could designate the Agg or
-	 * Unique node itself as the feature, but there shouldn't be any
-	 * RTI-bearing nodes between that node and this one.)
+	 * If this is a semijoin that was converted to an inner join by making
+	 * one side or the other unique, make a note that the inner or outer
+	 * subplan, as appropriate, should be treated as a query plan feature
+	 * when the main tree traversal reaches it.
+	 *
+	 * Conversely, if the planner could have made one side of the join unique
+	 * and thereby converted it to an inner join, and chose not to do so,
+	 * that is also worth noting. (XXX: I'm not sure that we need to emit
+	 * non-unique advice in every case where these join-type tests pass.)
+	 *
+	 * NB: This code could appear slightly higher up in in this function,
+	 * but none of the nodes through which we just descended should be have
+	 * associated RTIs.
+	 *
+	 * NB: This seems like a somewhat hacky way of passing information up
+	 * to the main tree walk, but I don't currently have a better idea.
 	 */
 	if (uniqueouter)
-		pgpa_add_future_feature(context, PGPAQF_SJ_UNIQUE, outerplan);
+		pgpa_add_future_feature(context, PGPAQF_SEMIJOIN_UNIQUE, outerplan);
+	else if (jointype == JOIN_RIGHT_SEMI)
+		pgpa_add_future_feature(context, PGPAQF_SEMIJOIN_NON_UNIQUE, outerplan);
 	if (uniqueinner)
-		pgpa_add_future_feature(context, PGPAQF_SJ_UNIQUE, innerplan);
+		pgpa_add_future_feature(context, PGPAQF_SEMIJOIN_UNIQUE, innerplan);
+	else if (jointype == JOIN_SEMI)
+		pgpa_add_future_feature(context, PGPAQF_SEMIJOIN_NON_UNIQUE, innerplan);
 
 	/* Set output parameters. */
 	*realouter = outerplan;
diff --git a/contrib/pg_plan_advice/pgpa_output.c b/contrib/pg_plan_advice/pgpa_output.c
index a8c121d9c8..69ca0c36f4 100644
--- a/contrib/pg_plan_advice/pgpa_output.c
+++ b/contrib/pg_plan_advice/pgpa_output.c
@@ -147,8 +147,11 @@ pgpa_output_advice(StringInfo buf, pgpa_plan_walker_context *walker,
 			case PGPAQF_GATHER_MERGE:
 				appendStringInfo(buf, "GATHER_MERGE(");
 				break;
-			case PGPAQF_SJ_UNIQUE:
-				appendStringInfo(buf, "SJ_UNIQUE(");
+			case PGPAQF_SEMIJOIN_NON_UNIQUE:
+				appendStringInfo(buf, "SEMIJOIN_NON_UNIQUE(");
+				break;
+			case PGPAQF_SEMIJOIN_UNIQUE:
+				appendStringInfo(buf, "SEMIJOIN_UNIQUE(");
 				break;
 		}
 		pgpa_output_relations(&context, buf, qf->relids);
diff --git a/contrib/pg_plan_advice/pgpa_walker.h b/contrib/pg_plan_advice/pgpa_walker.h
index 65a366ee06..223d42d746 100644
--- a/contrib/pg_plan_advice/pgpa_walker.h
+++ b/contrib/pg_plan_advice/pgpa_walker.h
@@ -23,11 +23,12 @@
  * For example, Gather nodes, desiginated by PGPAQF_GATHER, and Gather Merge
  * nodes, designated by PGPAQF_GATHER_MERGE, are query features, because we'll
  * want to admit some kind of advice that describes the portion of the plan
- * tree that appears beneath those nodes. Whenever we decide to implement a
- * semijoin by making one side unique and then performing a normal join, we
- * create a feature of PGPAQF_SG_UNIQUE, so that we can describe what RTIs need
- * to be made unique to perform the join. XXX We should also probably emit
- * advice for the decision NOT to unique-ify a semijoin.
+ * tree that appears beneath those nodes.
+ *
+ * Each semijoin can be implemented either by directly performing a semijoin,
+ * or by making one side unique and then performing a normal join. Either way,
+ * we use a query feature to notice what decision was made, so that we can
+ * describe it by enumerating the RTIs on that side of the join.
  *
  * To elaborate on the "no admixture of parent and child RTIs" rule, in all of
  * these cases, if the entirety of an inheritance hierarchy appears beneath
@@ -41,7 +42,8 @@ typedef enum pgpa_qf_type
 {
 	PGPAQF_GATHER,
 	PGPAQF_GATHER_MERGE,
-	PGPAQF_SJ_UNIQUE
+	PGPAQF_SEMIJOIN_NON_UNIQUE,
+	PGPAQF_SEMIJOIN_UNIQUE
 } pgpa_qf_type;
 
 /*