Update some comments that should've covered MERGE

Oversight in 7103ebb7aa.  Backpatch to 15.

Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAMbWs48gnDjZXq3-b56dVpQCNUJ5hD9kdtWN4QFwKCEapspNsA@mail.gmail.com

src/backend/optimizer/path/indxpath.c

@@ -3393,12 +3393,13 @@ check_index_predicates(PlannerInfo *root, RelOptInfo *rel)
 	 * Normally we remove quals that are implied by a partial index's
 	 * predicate from indrestrictinfo, indicating that they need not be
 	 * checked explicitly by an indexscan plan using this index.  However, if
-	 * the rel is a target relation of UPDATE/DELETE/SELECT FOR UPDATE, we
-	 * cannot remove such quals from the plan, because they need to be in the
-	 * plan so that they will be properly rechecked by EvalPlanQual testing.
-	 * Some day we might want to remove such quals from the main plan anyway
-	 * and pass them through to EvalPlanQual via a side channel; but for now,
-	 * we just don't remove implied quals at all for target relations.
+	 * the rel is a target relation of UPDATE/DELETE/MERGE/SELECT FOR UPDATE,
+	 * we cannot remove such quals from the plan, because they need to be in
+	 * the plan so that they will be properly rechecked by EvalPlanQual
+	 * testing.  Some day we might want to remove such quals from the main
+	 * plan anyway and pass them through to EvalPlanQual via a side channel;
+	 * but for now, we just don't remove implied quals at all for target
+	 * relations.
 	 */
 	is_target_rel = (bms_is_member(rel->relid, root->all_result_relids) ||
 					 get_plan_rowmark(root->rowMarks, rel->relid) != NULL);

src/backend/optimizer/plan/planmain.c

@@ -266,7 +266,7 @@ query_planner(PlannerInfo *root,
 	add_other_rels_to_query(root);
 
 	/*
-	 * Distribute any UPDATE/DELETE row identity variables to the target
+	 * Distribute any UPDATE/DELETE/MERGE row identity variables to the target
 	 * relations.  This can't be done till we've finished expansion of
 	 * appendrels.
 	 */

src/backend/optimizer/plan/planner.c

@@ -1749,7 +1749,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 
 			if (bms_membership(root->all_result_relids) == BMS_MULTIPLE)
 			{
-				/* Inherited UPDATE/DELETE */
+				/* Inherited UPDATE/DELETE/MERGE */
 				RelOptInfo *top_result_rel = find_base_rel(root,
 														   parse->resultRelation);
 				int			resultRelation = -1;
@@ -1876,7 +1876,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 			}
 			else
 			{
-				/* Single-relation INSERT/UPDATE/DELETE. */
+				/* Single-relation INSERT/UPDATE/DELETE/MERGE. */
 				resultRelations = list_make1_int(parse->resultRelation);
 				if (parse->commandType == CMD_UPDATE)
 					updateColnosLists = list_make1(root->update_colnos);

src/backend/optimizer/util/appendinfo.c

@@ -730,7 +730,7 @@ find_appinfos_by_relids(PlannerInfo *root, Relids relids, int *nappinfos)
 
 /*
  * add_row_identity_var
- *	  Register a row-identity column to be used in UPDATE/DELETE.
+ *	  Register a row-identity column to be used in UPDATE/DELETE/MERGE.
  *
  * The Var must be equal(), aside from varno, to any other row-identity
  * column with the same rowid_name.  Thus, for example, "wholerow"
@@ -909,8 +909,8 @@ add_row_identity_columns(PlannerInfo *root, Index rtindex,
  * distribute_row_identity_vars
  *
  * After we have finished identifying all the row identity columns
- * needed by an inherited UPDATE/DELETE query, make sure that these
- * columns will be generated by all the target relations.
+ * needed by an inherited UPDATE/DELETE/MERGE query, make sure that
+ * these columns will be generated by all the target relations.
  *
  * This is more or less like what build_base_rel_tlists() does,
  * except that it would not understand what to do with ROWID_VAR Vars.

src/backend/optimizer/util/inherit.c

@@ -609,7 +609,7 @@ expand_single_inheritance_child(PlannerInfo *root, RangeTblEntry *parentrte,
 
 	/*
 	 * If we are creating a child of the query target relation (only possible
-	 * in UPDATE/DELETE), add it to all_result_relids, as well as
+	 * in UPDATE/DELETE/MERGE), add it to all_result_relids, as well as
 	 * leaf_result_relids if appropriate, and make sure that we generate
 	 * required row-identity data.
 	 */

src/backend/optimizer/util/pathnode.c

@@ -3630,7 +3630,8 @@ create_lockrows_path(PlannerInfo *root, RelOptInfo *rel,
 
 /*
  * create_modifytable_path
- *	  Creates a pathnode that represents performing INSERT/UPDATE/DELETE mods
+ *	  Creates a pathnode that represents performing INSERT/UPDATE/DELETE/MERGE
+ *	  mods
  *
  * 'rel' is the parent relation associated with the result
  * 'subpath' is a Path producing source data

src/backend/optimizer/util/relnode.c

@@ -1013,7 +1013,7 @@ build_joinrel_tlist(PlannerInfo *root, RelOptInfo *joinrel,
 
 		if (var->varno == ROWID_VAR)
 		{
-			/* UPDATE/DELETE row identity vars are always needed */
+			/* UPDATE/DELETE/MERGE row identity vars are always needed */
 			RowIdentityVarInfo *ridinfo = (RowIdentityVarInfo *)
 			list_nth(root->row_identity_vars, var->varattno - 1);
 

src/backend/parser/parse_clause.c

@@ -153,7 +153,7 @@ transformFromClause(ParseState *pstate, List *frmList)
 
 /*
  * setTargetTable
- *	  Add the target relation of INSERT/UPDATE/DELETE to the range table,
+ *	  Add the target relation of INSERT/UPDATE/DELETE/MERGE to the range table,
  *	  and make the special links to it in the ParseState.
  *
  *	  We also open the target relation and acquire a write lock on it.
@@ -163,7 +163,9 @@ transformFromClause(ParseState *pstate, List *frmList)
  *
  *	  If alsoSource is true, add the target to the query's joinlist and
  *	  namespace.  For INSERT, we don't want the target to be joined to;
- *	  it's a destination of tuples, not a source.   For UPDATE/DELETE,
+ *	  it's a destination of tuples, not a source.  MERGE is actually
+ *	  both, but we'll add it separately to joinlist and namespace, so
+ *	  doing nothing (like INSERT) is correct here.  For UPDATE/DELETE,
  *	  we do need to scan or join the target.  (NOTE: we do not bother
  *	  to check for namespace conflict; we assume that the namespace was
  *	  initially empty in these cases.)

src/backend/parser/parse_expr.c

@@ -1679,8 +1679,8 @@ transformSubLink(ParseState *pstate, SubLink *sublink)
 
 	/*
 	 * Check to see if the sublink is in an invalid place within the query. We
-	 * allow sublinks everywhere in SELECT/INSERT/UPDATE/DELETE, but generally
-	 * not in utility statements.
+	 * allow sublinks everywhere in SELECT/INSERT/UPDATE/DELETE/MERGE, but
+	 * generally not in utility statements.
 	 */
 	err = NULL;
 	switch (pstate->p_expr_kind)

src/backend/parser/parse_merge.c

@@ -163,7 +163,11 @@ transformMergeStmt(ParseState *pstate, MergeStmt *stmt)
 					 errmsg("unreachable WHEN clause specified after unconditional WHEN clause")));
 	}
 
-	/* Set up the MERGE target table. */
+	/*
+	 * Set up the MERGE target table.  The target table is added to the
+	 * namespace below and to joinlist in transform_MERGE_to_join, so don't
+	 * do it here.
+	 */
 	qry->resultRelation = setTargetTable(pstate, stmt->relation,
 										 stmt->relation->inh,
 										 false, targetPerms);