Add support for MERGE SQL command

MERGE performs actions that modify rows in the target table using a
source table or query. MERGE provides a single SQL statement that can
conditionally INSERT/UPDATE/DELETE rows -- a task that would otherwise
require multiple PL statements.  For example,

MERGE INTO target AS t
USING source AS s
ON t.tid = s.sid
WHEN MATCHED AND t.balance > s.delta THEN
  UPDATE SET balance = t.balance - s.delta
WHEN MATCHED THEN
  DELETE
WHEN NOT MATCHED AND s.delta > 0 THEN
  INSERT VALUES (s.sid, s.delta)
WHEN NOT MATCHED THEN
  DO NOTHING;

MERGE works with regular tables, partitioned tables and inheritance
hierarchies, including column and row security enforcement, as well as
support for row and statement triggers and transition tables therein.

MERGE is optimized for OLTP and is parameterizable, though also useful
for large scale ETL/ELT. MERGE is not intended to be used in preference
to existing single SQL commands for INSERT, UPDATE or DELETE since there
is some overhead.  MERGE can be used from PL/pgSQL.

MERGE does not support targetting updatable views or foreign tables, and
RETURNING clauses are not allowed either.  These limitations are likely
fixable with sufficient effort.  Rewrite rules are also not supported,
but it's not clear that we'd want to support them.

Author: Pavan Deolasee <pavan.deolasee@gmail.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Author: Amit Langote <amitlangote09@gmail.com>
Author: Simon Riggs <simon.riggs@enterprisedb.com>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Reviewed-by: Andres Freund <andres@anarazel.de> (earlier versions)
Reviewed-by: Peter Geoghegan <pg@bowt.ie> (earlier versions)
Reviewed-by: Robert Haas <robertmhaas@gmail.com> (earlier versions)
Reviewed-by: Japin Li <japinli@hotmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Reviewed-by: Zhihong Yu <zyu@yugabyte.com>
Discussion: https://postgr.es/m/CANP8+jKitBSrB7oTgT9CY2i1ObfOt36z0XMraQc+Xrz8QB0nXA@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-WzkJdBuxj9PO=2QaO9-3h3xGbQPZ34kJH=HukRekwM-GZg@mail.gmail.com
Discussion: https://postgr.es/m/20201231134736.GA25392@alvherre.pgsql

1 files changed, 35 insertions, 1 deletions

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 44dd73fc80e..cbbcff81d2e 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -389,6 +389,22 @@ typedef struct OnConflictSetState
 	ExprState  *oc_WhereClause; /* state for the WHERE clause */
 } OnConflictSetState;
 
+/* ----------------
+ *	 MergeActionState information
+ *
+ *	Executor state for a MERGE action.
+ * ----------------
+ */
+typedef struct MergeActionState
+{
+	NodeTag		type;
+
+	MergeAction *mas_action;	/* associated MergeAction node */
+	ProjectionInfo *mas_proj;	/* projection of the action's targetlist for
+								 * this rel */
+	ExprState  *mas_whenqual;	/* WHEN [NOT] MATCHED AND conditions */
+} MergeActionState;
+
 /*
  * ResultRelInfo
  *
@@ -500,6 +516,10 @@ typedef struct ResultRelInfo
 	/* ON CONFLICT evaluation state */
 	OnConflictSetState *ri_onConflict;
 
+	/* for MERGE, lists of MergeActionState */
+	List	   *ri_matchedMergeAction;
+	List	   *ri_notMatchedMergeAction;
+
 	/* partition check expression state (NULL if not set up yet) */
 	ExprState  *ri_PartitionCheckExpr;
 
@@ -1190,6 +1210,12 @@ typedef struct ProjectSetState
 	MemoryContext argcontext;	/* context for SRF arguments */
 } ProjectSetState;
 
+
+/* flags for mt_merge_subcommands */
+#define MERGE_INSERT	0x01
+#define MERGE_UPDATE	0x02
+#define MERGE_DELETE	0x04
+
 /* ----------------
  *	 ModifyTableState information
  * ----------------
@@ -1197,7 +1223,7 @@ typedef struct ProjectSetState
 typedef struct ModifyTableState
 {
 	PlanState	ps;				/* its first field is NodeTag */
-	CmdType		operation;		/* INSERT, UPDATE, or DELETE */
+	CmdType		operation;		/* INSERT, UPDATE, DELETE, or MERGE */
 	bool		canSetTag;		/* do we set the command tag/es_processed? */
 	bool		mt_done;		/* are we done? */
 	int			mt_nrels;		/* number of entries in resultRelInfo[] */
@@ -1239,6 +1265,14 @@ typedef struct ModifyTableState
 
 	/* controls transition table population for INSERT...ON CONFLICT UPDATE */
 	struct TransitionCaptureState *mt_oc_transition_capture;
+
+	/* Flags showing which subcommands are present INS/UPD/DEL/DO NOTHING */
+	int			mt_merge_subcommands;
+
+	/* tuple counters for MERGE */
+	double		mt_merge_inserted;
+	double		mt_merge_updated;
+	double		mt_merge_deleted;
 } ModifyTableState;
 
 /* ----------------