10 files changed, 37 insertions, 2 deletions
diff --git a/src/include/catalog/catversion.h b/src/include/catalog/catversion.h
index aec49079c12..43a9a707094 100644
--- a/src/include/catalog/catversion.h
+++ b/src/include/catalog/catversion.h
@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202403171
+#define CATALOG_VERSION_NO	202403172
 
 #endif
diff --git a/src/include/executor/execExpr.h b/src/include/executor/execExpr.h
index a28ddcdd771..8953d767387 100644
--- a/src/include/executor/execExpr.h
+++ b/src/include/executor/execExpr.h
@@ -243,6 +243,7 @@ typedef enum ExprEvalOp
 	EEOP_AGGREF,
 	EEOP_GROUPING_FUNC,
 	EEOP_WINDOW_FUNC,
+	EEOP_MERGE_SUPPORT_FUNC,
 	EEOP_SUBPLAN,
 
 	/* aggregation related nodes */
@@ -810,6 +811,8 @@ extern void ExecEvalJsonConstructor(ExprState *state, ExprEvalStep *op,
 									ExprContext *econtext);
 extern void ExecEvalJsonIsPredicate(ExprState *state, ExprEvalStep *op);
 extern void ExecEvalGroupingFunc(ExprState *state, ExprEvalStep *op);
+extern void ExecEvalMergeSupportFunc(ExprState *state, ExprEvalStep *op,
+									 ExprContext *econtext);
 extern void ExecEvalSubPlan(ExprState *state, ExprEvalStep *op,
 							ExprContext *econtext);
 extern void ExecEvalWholeRowVar(ExprState *state, ExprEvalStep *op,
diff --git a/src/include/executor/spi.h b/src/include/executor/spi.h
index c13e1f64fb1..48b87730ea0 100644
--- a/src/include/executor/spi.h
+++ b/src/include/executor/spi.h
@@ -97,6 +97,7 @@ typedef struct _SPI_plan *SPIPlanPtr;
 #define SPI_OK_REL_UNREGISTER	16
 #define SPI_OK_TD_REGISTER		17
 #define SPI_OK_MERGE			18
+#define SPI_OK_MERGE_RETURNING	19
 
 #define SPI_OPT_NONATOMIC		(1 << 0)
 
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 27614ab50fb..92593526725 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1325,6 +1325,9 @@ typedef struct ModifyTableState
 	/* Flags showing which subcommands are present INS/UPD/DEL/DO NOTHING */
 	int			mt_merge_subcommands;
 
+	/* For MERGE, the action currently being executed */
+	MergeActionState *mt_merge_action;
+
 	/* tuple counters for MERGE */
 	double		mt_merge_inserted;
 	double		mt_merge_updated;
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 70a21df0fee..7b57fddf2d0 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -1939,6 +1939,7 @@ typedef struct MergeStmt
 	Node	   *sourceRelation; /* source relation */
 	Node	   *joinCondition;	/* join condition between source and target */
 	List	   *mergeWhenClauses;	/* list of MergeWhenClause(es) */
+	List	   *returningList;	/* list of expressions to return */
 	WithClause *withClause;		/* WITH clause */
 } MergeStmt;
 
diff --git a/src/include/nodes/primnodes.h b/src/include/nodes/primnodes.h
index 4a154606d2b..8df8884001d 100644
--- a/src/include/nodes/primnodes.h
+++ b/src/include/nodes/primnodes.h
@@ -572,6 +572,27 @@ typedef struct WindowFunc
 } WindowFunc;
 
 /*
+ * MergeSupportFunc
+ *
+ * A MergeSupportFunc is a merge support function expression that can only
+ * appear in the RETURNING list of a MERGE command.  It returns information
+ * about the currently executing merge action.
+ *
+ * Currently, the only supported function is MERGE_ACTION(), which returns the
+ * command executed ("INSERT", "UPDATE", or "DELETE").
+ */
+typedef struct MergeSupportFunc
+{
+	Expr		xpr;
+	/* type Oid of result */
+	Oid			msftype;
+	/* OID of collation, or InvalidOid if none */
+	Oid			msfcollid;
+	/* token location, or -1 if unknown */
+	int			location;
+} MergeSupportFunc;
+
+/*
  * SubscriptingRef: describes a subscripting operation over a container
  * (array, etc).
  *
diff --git a/src/include/optimizer/paramassign.h b/src/include/optimizer/paramassign.h
index 501f728d714..4026b74fab4 100644
--- a/src/include/optimizer/paramassign.h
+++ b/src/include/optimizer/paramassign.h
@@ -20,6 +20,8 @@ extern Param *replace_outer_placeholdervar(PlannerInfo *root,
 										   PlaceHolderVar *phv);
 extern Param *replace_outer_agg(PlannerInfo *root, Aggref *agg);
 extern Param *replace_outer_grouping(PlannerInfo *root, GroupingFunc *grp);
+extern Param *replace_outer_merge_support(PlannerInfo *root,
+										  MergeSupportFunc *msf);
 extern Param *replace_nestloop_param_var(PlannerInfo *root, Var *var);
 extern Param *replace_nestloop_param_placeholdervar(PlannerInfo *root,
 													PlaceHolderVar *phv);
diff --git a/src/include/parser/analyze.h b/src/include/parser/analyze.h
index 5ecdd109466..28b66fccb43 100644
--- a/src/include/parser/analyze.h
+++ b/src/include/parser/analyze.h
@@ -44,6 +44,8 @@ extern List *transformInsertRow(ParseState *pstate, List *exprlist,
 								bool strip_indirection);
 extern List *transformUpdateTargetList(ParseState *pstate,
 									   List *origTlist);
+extern List *transformReturningList(ParseState *pstate, List *returningList,
+									ParseExprKind exprKind);
 extern Query *transformTopLevelStmt(ParseState *pstate, RawStmt *parseTree);
 extern Query *transformStmt(ParseState *pstate, Node *parseTree);
 
diff --git a/src/include/parser/kwlist.h b/src/include/parser/kwlist.h
index 2331acac091..099353469b5 100644
--- a/src/include/parser/kwlist.h
+++ b/src/include/parser/kwlist.h
@@ -266,6 +266,7 @@ PG_KEYWORD("matched", MATCHED, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("materialized", MATERIALIZED, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("maxvalue", MAXVALUE, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("merge", MERGE, UNRESERVED_KEYWORD, BARE_LABEL)
+PG_KEYWORD("merge_action", MERGE_ACTION, COL_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("method", METHOD, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("minute", MINUTE_P, UNRESERVED_KEYWORD, AS_LABEL)
 PG_KEYWORD("minvalue", MINVALUE, UNRESERVED_KEYWORD, BARE_LABEL)
diff --git a/src/include/parser/parse_node.h b/src/include/parser/parse_node.h
index 99d65157365..5b781d87a9d 100644
--- a/src/include/parser/parse_node.h
+++ b/src/include/parser/parse_node.h
@@ -61,7 +61,8 @@ typedef enum ParseExprKind
 	EXPR_KIND_DISTINCT_ON,		/* DISTINCT ON */
 	EXPR_KIND_LIMIT,			/* LIMIT */
 	EXPR_KIND_OFFSET,			/* OFFSET */
-	EXPR_KIND_RETURNING,		/* RETURNING */
+	EXPR_KIND_RETURNING,		/* RETURNING in INSERT/UPDATE/DELETE */
+	EXPR_KIND_MERGE_RETURNING,	/* RETURNING in MERGE */
 	EXPR_KIND_VALUES,			/* VALUES */
 	EXPR_KIND_VALUES_SINGLE,	/* single-row VALUES (in INSERT only) */
 	EXPR_KIND_CHECK_CONSTRAINT, /* CHECK constraint for a table */