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);

src/include/nodes/parsenodes.h

@@ -1043,8 +1043,8 @@ typedef struct RangeTblEntry
 	 *
 	 * rellockmode is really LOCKMODE, but it's declared int to avoid having
 	 * to include lock-related headers here.  It must be RowExclusiveLock if
-	 * the RTE is an INSERT/UPDATE/DELETE target, else RowShareLock if the RTE
-	 * is a SELECT FOR UPDATE/FOR SHARE target, else AccessShareLock.
+	 * the RTE is an INSERT/UPDATE/DELETE/MERGE target, else RowShareLock if
+	 * the RTE is a SELECT FOR UPDATE/FOR SHARE target, else AccessShareLock.
 	 *
 	 * Note: in some cases, rule expansion may result in RTEs that are marked
 	 * with RowExclusiveLock even though they are not the target of the

src/include/nodes/pathnodes.h

@@ -328,11 +328,11 @@ struct PlannerInfo
 
 	/*
 	 * all_result_relids is empty for SELECT, otherwise it contains at least
-	 * parse->resultRelation.  For UPDATE/DELETE across an inheritance or
-	 * partitioning tree, the result rel's child relids are added.  When using
-	 * multi-level partitioning, intermediate partitioned rels are included.
-	 * leaf_result_relids is similar except that only actual result tables,
-	 * not partitioned tables, are included in it.
+	 * parse->resultRelation.  For UPDATE/DELETE/MERGE across an inheritance
+	 * or partitioning tree, the result rel's child relids are added.  When
+	 * using multi-level partitioning, intermediate partitioned rels are
+	 * included. leaf_result_relids is similar except that only actual result
+	 * tables, not partitioned tables, are included in it.
 	 */
 	/* set of all result relids */
 	Relids		all_result_relids;
@@ -2792,10 +2792,10 @@ typedef struct AppendRelInfo
 } AppendRelInfo;
 
 /*
- * Information about a row-identity "resjunk" column in UPDATE/DELETE.
+ * Information about a row-identity "resjunk" column in UPDATE/DELETE/MERGE.
  *
- * In partitioned UPDATE/DELETE it's important for child partitions to share
- * row-identity columns whenever possible, so as not to chew up too many
+ * In partitioned UPDATE/DELETE/MERGE it's important for child partitions to
+ * share row-identity columns whenever possible, so as not to chew up too many
  * targetlist columns.  We use these structs to track which identity columns
  * have been requested.  In the finished plan, each of these will give rise
  * to one resjunk entry in the targetlist of the ModifyTable's subplan node.

src/include/nodes/plannodes.h

@@ -72,7 +72,7 @@ typedef struct PlannedStmt
 
 	List	   *rtable;			/* list of RangeTblEntry nodes */
 
-	/* rtable indexes of target relations for INSERT/UPDATE/DELETE */
+	/* rtable indexes of target relations for INSERT/UPDATE/DELETE/MERGE */
 	List	   *resultRelations;	/* integer list of RT indexes, or NIL */
 
 	List	   *appendRelations;	/* list of AppendRelInfo nodes */
@@ -217,7 +217,7 @@ typedef struct ProjectSet
  * nominalRelation and rootRelation contain the RT index of the partition
  * root, which is not otherwise mentioned in the plan.  Otherwise rootRelation
  * is zero.  However, nominalRelation will always be set, as it's the rel that
- * EXPLAIN should claim is the INSERT/UPDATE/DELETE target.
+ * EXPLAIN should claim is the INSERT/UPDATE/DELETE/MERGE target.
  *
  * Note that rowMarks and epqParam are presumed to be valid for all the
  * table(s); they can't contain any info that varies across tables.

src/include/nodes/primnodes.h

@@ -177,8 +177,8 @@ typedef struct Expr
  * is abused to signify references to columns of a custom scan tuple type.)
  *
  * ROWID_VAR is used in the planner to identify nonce variables that carry
- * row identity information during UPDATE/DELETE.  This value should never
- * be seen outside the planner.
+ * row identity information during UPDATE/DELETE/MERGE.  This value should
+ * never be seen outside the planner.
  *
  * In the parser, varnosyn and varattnosyn are either identical to
  * varno/varattno, or they specify the column's position in an aliased JOIN