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, 57 insertions, 4 deletions

diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 3b12a708ec0..6bf212b01a1 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -119,7 +119,7 @@ typedef struct Query
 {
 	NodeTag		type;
 
-	CmdType		commandType;	/* select|insert|update|delete|utility */
+	CmdType		commandType;	/* select|insert|update|delete|merge|utility */
 
 	QuerySource querySource;	/* where did I come from? */
 
@@ -130,7 +130,7 @@ typedef struct Query
 	Node	   *utilityStmt;	/* non-null if commandType == CMD_UTILITY */
 
 	int			resultRelation; /* rtable index of target relation for
-								 * INSERT/UPDATE/DELETE; 0 for SELECT */
+								 * INSERT/UPDATE/DELETE/MERGE; 0 for SELECT */
 
 	bool		hasAggs;		/* has aggregates in tlist or havingQual */
 	bool		hasWindowFuncs; /* has window functions in tlist */
@@ -147,7 +147,11 @@ typedef struct Query
 	List	   *cteList;		/* WITH list (of CommonTableExpr's) */
 
 	List	   *rtable;			/* list of range table entries */
-	FromExpr   *jointree;		/* table join tree (FROM and WHERE clauses) */
+	FromExpr   *jointree;		/* table join tree (FROM and WHERE clauses);
+								 * also USING clause for MERGE */
+
+	List	   *mergeActionList;	/* list of actions for MERGE (only) */
+	bool		mergeUseOuterJoin;	/* whether to use outer join */
 
 	List	   *targetList;		/* target list (of TargetEntry) */
 
@@ -1221,7 +1225,9 @@ typedef enum WCOKind
 	WCO_VIEW_CHECK,				/* WCO on an auto-updatable view */
 	WCO_RLS_INSERT_CHECK,		/* RLS INSERT WITH CHECK policy */
 	WCO_RLS_UPDATE_CHECK,		/* RLS UPDATE WITH CHECK policy */
-	WCO_RLS_CONFLICT_CHECK		/* RLS ON CONFLICT DO UPDATE USING policy */
+	WCO_RLS_CONFLICT_CHECK,		/* RLS ON CONFLICT DO UPDATE USING policy */
+	WCO_RLS_MERGE_UPDATE_CHECK, /* RLS MERGE UPDATE USING policy */
+	WCO_RLS_MERGE_DELETE_CHECK	/* RLS MERGE DELETE USING policy */
 } WCOKind;
 
 typedef struct WithCheckOption
@@ -1538,6 +1544,39 @@ typedef struct CommonTableExpr
 	 ((Query *) (cte)->ctequery)->returningList)
 
 /*
+ * MergeWhenClause -
+ *		raw parser representation of a WHEN clause in a MERGE statement
+ *
+ * This is transformed into MergeAction by parse analysis
+ */
+typedef struct MergeWhenClause
+{
+	NodeTag		type;
+	bool		matched;		/* true=MATCHED, false=NOT MATCHED */
+	CmdType		commandType;	/* INSERT/UPDATE/DELETE/DO NOTHING */
+	OverridingKind override;	/* OVERRIDING clause */
+	Node	   *condition;		/* WHEN conditions (raw parser) */
+	List	   *targetList;		/* INSERT/UPDATE targetlist */
+	/* the following members are only used in INSERT actions */
+	List	   *values;			/* VALUES to INSERT, or NULL */
+} MergeWhenClause;
+
+/*
+ * MergeAction -
+ *		Transformed representation of a WHEN clause in a MERGE statement
+ */
+typedef struct MergeAction
+{
+	NodeTag		type;
+	bool		matched;		/* true=MATCHED, false=NOT MATCHED */
+	CmdType		commandType;	/* INSERT/UPDATE/DELETE/DO NOTHING */
+	OverridingKind override;	/* OVERRIDING clause */
+	Node	   *qual;			/* transformed WHEN conditions */
+	List	   *targetList;		/* the target list (of TargetEntry) */
+	List	   *updateColnos;	/* target attribute numbers of an UPDATE */
+} MergeAction;
+
+/*
  * TriggerTransition -
  *	   representation of transition row or table naming clause
  *
@@ -1738,6 +1777,20 @@ typedef struct UpdateStmt
 } UpdateStmt;
 
 /* ----------------------
+ *		Merge Statement
+ * ----------------------
+ */
+typedef struct MergeStmt
+{
+	NodeTag		type;
+	RangeVar   *relation;		/* target relation to merge into */
+	Node	   *sourceRelation; /* source relation */
+	Node	   *joinCondition;	/* join condition between source and target */
+	List	   *mergeWhenClauses;	/* list of MergeWhenClause(es) */
+	WithClause *withClause;		/* WITH clause */
+} MergeStmt;
+
+/* ----------------------
  *		Select Statement
  *
  * A "simple" SELECT is represented in the output of gram.y by a single