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, 16 insertions, 0 deletions

diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
index 3780c6e812e..f364a9b88a9 100644
--- a/src/backend/tcop/utility.c
+++ b/src/backend/tcop/utility.c
@@ -113,6 +113,7 @@ CommandIsReadOnly(PlannedStmt *pstmt)
 		case CMD_UPDATE:
 		case CMD_INSERT:
 		case CMD_DELETE:
+		case CMD_MERGE:
 			return false;
 		case CMD_UTILITY:
 			/* For now, treat all utility commands as read/write */
@@ -2124,6 +2125,8 @@ QueryReturnsTuples(Query *parsetree)
 		case CMD_SELECT:
 			/* returns tuples */
 			return true;
+		case CMD_MERGE:
+			return false;
 		case CMD_INSERT:
 		case CMD_UPDATE:
 		case CMD_DELETE:
@@ -2365,6 +2368,10 @@ CreateCommandTag(Node *parsetree)
 			tag = CMDTAG_UPDATE;
 			break;
 
+		case T_MergeStmt:
+			tag = CMDTAG_MERGE;
+			break;
+
 		case T_SelectStmt:
 			tag = CMDTAG_SELECT;
 			break;
@@ -3125,6 +3132,9 @@ CreateCommandTag(Node *parsetree)
 					case CMD_DELETE:
 						tag = CMDTAG_DELETE;
 						break;
+					case CMD_MERGE:
+						tag = CMDTAG_MERGE;
+						break;
 					case CMD_UTILITY:
 						tag = CreateCommandTag(stmt->utilityStmt);
 						break;
@@ -3185,6 +3195,9 @@ CreateCommandTag(Node *parsetree)
 					case CMD_DELETE:
 						tag = CMDTAG_DELETE;
 						break;
+					case CMD_MERGE:
+						tag = CMDTAG_MERGE;
+						break;
 					case CMD_UTILITY:
 						tag = CreateCommandTag(stmt->utilityStmt);
 						break;
@@ -3233,6 +3246,7 @@ GetCommandLogLevel(Node *parsetree)
 		case T_InsertStmt:
 		case T_DeleteStmt:
 		case T_UpdateStmt:
+		case T_MergeStmt:
 			lev = LOGSTMT_MOD;
 			break;
 
@@ -3682,6 +3696,7 @@ GetCommandLogLevel(Node *parsetree)
 					case CMD_UPDATE:
 					case CMD_INSERT:
 					case CMD_DELETE:
+					case CMD_MERGE:
 						lev = LOGSTMT_MOD;
 						break;
 
@@ -3712,6 +3727,7 @@ GetCommandLogLevel(Node *parsetree)
 					case CMD_UPDATE:
 					case CMD_INSERT:
 					case CMD_DELETE:
+					case CMD_MERGE:
 						lev = LOGSTMT_MOD;
 						break;