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

4 files changed, 15 insertions, 3 deletions

diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index 915139378e..00328fddcf 100644
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/src/pl/plpgsql/src/pl_exec.c
@@ -4194,7 +4194,7 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 
 	/*
 	 * On the first call for this statement generate the plan, and detect
-	 * whether the statement is INSERT/UPDATE/DELETE
+	 * whether the statement is INSERT/UPDATE/DELETE/MERGE
 	 */
 	if (expr->plan == NULL)
 		exec_prepare_plan(estate, expr, CURSOR_OPT_PARALLEL_OK);
@@ -4216,7 +4216,8 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 			 */
 			if (plansource->commandTag == CMDTAG_INSERT ||
 				plansource->commandTag == CMDTAG_UPDATE ||
-				plansource->commandTag == CMDTAG_DELETE)
+				plansource->commandTag == CMDTAG_DELETE ||
+				plansource->commandTag == CMDTAG_MERGE)
 			{
 				stmt->mod_stmt = true;
 				break;
@@ -4276,6 +4277,7 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 		case SPI_OK_INSERT_RETURNING:
 		case SPI_OK_UPDATE_RETURNING:
 		case SPI_OK_DELETE_RETURNING:
+		case SPI_OK_MERGE:
 			Assert(stmt->mod_stmt);
 			exec_set_found(estate, (SPI_processed != 0));
 			break;
@@ -4457,6 +4459,7 @@ exec_stmt_dynexecute(PLpgSQL_execstate *estate,
 		case SPI_OK_INSERT_RETURNING:
 		case SPI_OK_UPDATE_RETURNING:
 		case SPI_OK_DELETE_RETURNING:
+		case SPI_OK_MERGE:
 		case SPI_OK_UTILITY:
 		case SPI_OK_REWRITTEN:
 			break;
diff --git a/src/pl/plpgsql/src/pl_gram.y b/src/pl/plpgsql/src/pl_gram.y
index 954c2df331..11e86c1609 100644
--- a/src/pl/plpgsql/src/pl_gram.y
+++ b/src/pl/plpgsql/src/pl_gram.y
@@ -306,6 +306,7 @@ static	void			check_raise_parameters(PLpgSQL_stmt_raise *stmt);
 %token <keyword>	K_LAST
 %token <keyword>	K_LOG
 %token <keyword>	K_LOOP
+%token <keyword>	K_MERGE
 %token <keyword>	K_MESSAGE
 %token <keyword>	K_MESSAGE_TEXT
 %token <keyword>	K_MOVE
@@ -2000,6 +2001,10 @@ stmt_execsql	: K_IMPORT
 					{
 						$$ = make_execsql_stmt(K_INSERT, @1);
 					}
+				| K_MERGE
+					{
+						$$ = make_execsql_stmt(K_MERGE, @1);
+					}
 				| T_WORD
 					{
 						int			tok;
@@ -2537,6 +2542,7 @@ unreserved_keyword	:
 				| K_IS
 				| K_LAST
 				| K_LOG
+				| K_MERGE
 				| K_MESSAGE
 				| K_MESSAGE_TEXT
 				| K_MOVE
@@ -3000,6 +3006,8 @@ make_execsql_stmt(int firsttoken, int location)
 		{
 			if (prev_tok == K_INSERT)
 				continue;		/* INSERT INTO is not an INTO-target */
+			if (prev_tok == K_MERGE)
+				continue;		/* MERGE INTO is not an INTO-target */
 			if (firsttoken == K_IMPORT)
 				continue;		/* IMPORT ... INTO is not an INTO-target */
 			if (have_into)
diff --git a/src/pl/plpgsql/src/pl_unreserved_kwlist.h b/src/pl/plpgsql/src/pl_unreserved_kwlist.h
index 1c68420331..ee2be1b212 100644
--- a/src/pl/plpgsql/src/pl_unreserved_kwlist.h
+++ b/src/pl/plpgsql/src/pl_unreserved_kwlist.h
@@ -70,6 +70,7 @@ PG_KEYWORD("insert", K_INSERT)
 PG_KEYWORD("is", K_IS)
 PG_KEYWORD("last", K_LAST)
 PG_KEYWORD("log", K_LOG)
+PG_KEYWORD("merge", K_MERGE)
 PG_KEYWORD("message", K_MESSAGE)
 PG_KEYWORD("message_text", K_MESSAGE_TEXT)
 PG_KEYWORD("move", K_MOVE)
diff --git a/src/pl/plpgsql/src/plpgsql.h b/src/pl/plpgsql/src/plpgsql.h
index 18a4f6c7d3..813c32c70f 100644
--- a/src/pl/plpgsql/src/plpgsql.h
+++ b/src/pl/plpgsql/src/plpgsql.h
@@ -893,7 +893,7 @@ typedef struct PLpgSQL_stmt_execsql
 	int			lineno;
 	unsigned int stmtid;
 	PLpgSQL_expr *sqlstmt;
-	bool		mod_stmt;		/* is the stmt INSERT/UPDATE/DELETE? */
+	bool		mod_stmt;		/* is the stmt INSERT/UPDATE/DELETE/MERGE? */
 	bool		mod_stmt_set;	/* is mod_stmt valid yet? */
 	bool		into;			/* INTO supplied? */
 	bool		strict;			/* INTO STRICT flag */