Revert MERGE patch

This reverts commits d204ef6377,
83454e3c2b and a few more commits thereafter
(complete list at the end) related to MERGE feature.

While the feature was fully functional, with sufficient test coverage and
necessary documentation, it was felt that some parts of the executor and
parse-analyzer can use a different design and it wasn't possible to do that in
the available time. So it was decided to revert the patch for PG11 and retry
again in the future.

Thanks again to all reviewers and bug reporters.

List of commits reverted, in reverse chronological order:

 f1464c5380 Improve parse representation for MERGE
 ddb4158579 MERGE syntax diagram correction
 530e69e59b Allow cpluspluscheck to pass by renaming variable
 01b88b4df5 MERGE minor errata
 3af7b2b0d4 MERGE fix variable warning in non-assert builds
 a5d86181ec MERGE INSERT allows only one VALUES clause
 4b2d44031f MERGE post-commit review
 4923550c20 Tab completion for MERGE
 aa3faa3c7a WITH support in MERGE
 83454e3c2b New files for MERGE
 d204ef6377 MERGE SQL Command following SQL:2016

Author: Pavan Deolasee
Reviewed-by: Michael Paquier

src/include/parser/analyze.h

@@ -32,11 +32,6 @@ extern Query *parse_sub_analyze(Node *parseTree, ParseState *parentParseState,
 				  bool locked_from_parent,
 				  bool resolve_unknowns);
 
-extern List *transformInsertRow(ParseState *pstate, List *exprlist,
-				   List *stmtcols, List *icolumns, List *attrnos,
-				   bool strip_indirection);
-extern List *transformUpdateTargetList(ParseState *pstate,
-						  List *targetList);
 extern Query *transformTopLevelStmt(ParseState *pstate, RawStmt *parseTree);
 extern Query *transformStmt(ParseState *pstate, Node *parseTree);
 

src/include/parser/kwlist.h

@@ -245,10 +245,8 @@ PG_KEYWORD("locked", LOCKED, UNRESERVED_KEYWORD)
 PG_KEYWORD("logged", LOGGED, UNRESERVED_KEYWORD)
 PG_KEYWORD("mapping", MAPPING, UNRESERVED_KEYWORD)
 PG_KEYWORD("match", MATCH, UNRESERVED_KEYWORD)
-PG_KEYWORD("matched", MATCHED, UNRESERVED_KEYWORD)
 PG_KEYWORD("materialized", MATERIALIZED, UNRESERVED_KEYWORD)
 PG_KEYWORD("maxvalue", MAXVALUE, UNRESERVED_KEYWORD)
-PG_KEYWORD("merge", MERGE, UNRESERVED_KEYWORD)
 PG_KEYWORD("method", METHOD, UNRESERVED_KEYWORD)
 PG_KEYWORD("minute", MINUTE_P, UNRESERVED_KEYWORD)
 PG_KEYWORD("minvalue", MINVALUE, UNRESERVED_KEYWORD)

src/include/parser/parse_clause.h

@@ -20,10 +20,7 @@ extern void transformFromClause(ParseState *pstate, List *frmList);
 extern int setTargetTable(ParseState *pstate, RangeVar *relation,
 			   bool inh, bool alsoSource, AclMode requiredPerms);
 extern bool interpretOidsOption(List *defList, bool allowOids);
-extern Node *transformFromClauseItem(ParseState *pstate, Node *n,
-						RangeTblEntry **top_rte, int *top_rti,
-						RangeTblEntry **right_rte, int *right_rti,
-						List **fnamespace);
+
 extern Node *transformWhereClause(ParseState *pstate, Node *clause,
 					 ParseExprKind exprKind, const char *constructName);
 extern Node *transformLimitClause(ParseState *pstate, Node *clause,

src/include/parser/parse_merge.h

@@ -1,19 +0,0 @@
-/*-------------------------------------------------------------------------
- *
- * parse_merge.h
- *	  handle merge-stmt in parser
- *
- *
- * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
- * Portions Copyright (c) 1994, Regents of the University of California
- *
- * src/include/parser/parse_merge.h
- *
- *-------------------------------------------------------------------------
- */
-#ifndef PARSE_MERGE_H
-#define PARSE_MERGE_H
-
-#include "parser/parse_node.h"
-extern Query *transformMergeStmt(ParseState *pstate, MergeStmt *stmt);
-#endif

src/include/parser/parse_node.h

@@ -50,7 +50,6 @@ typedef enum ParseExprKind
 	EXPR_KIND_INSERT_TARGET,	/* INSERT target list item */
 	EXPR_KIND_UPDATE_SOURCE,	/* UPDATE assignment source item */
 	EXPR_KIND_UPDATE_TARGET,	/* UPDATE assignment target item */
-	EXPR_KIND_MERGE_WHEN_AND,	/* MERGE WHEN ... AND condition */
 	EXPR_KIND_GROUP_BY,			/* GROUP BY */
 	EXPR_KIND_ORDER_BY,			/* ORDER BY */
 	EXPR_KIND_DISTINCT_ON,		/* DISTINCT ON */
@@ -128,7 +127,7 @@ typedef Node *(*CoerceParamHook) (ParseState *pstate, Param *param,
  * p_parent_cte: CommonTableExpr that immediately contains the current query,
  * if any.
  *
- * p_target_relation: target relation, if query is INSERT/UPDATE/DELETE/MERGE
+ * p_target_relation: target relation, if query is INSERT, UPDATE, or DELETE.
  *
  * p_target_rangetblentry: target relation's entry in the rtable list.
  *
@@ -182,7 +181,7 @@ struct ParseState
 	List	   *p_ctenamespace; /* current namespace for common table exprs */
 	List	   *p_future_ctes;	/* common table exprs not yet in namespace */
 	CommonTableExpr *p_parent_cte;	/* this query's containing CTE */
-	Relation	p_target_relation;	/* INSERT/UPDATE/DELETE/MERGE target rel */
+	Relation	p_target_relation;	/* INSERT/UPDATE/DELETE target rel */
 	RangeTblEntry *p_target_rangetblentry;	/* target rel's RTE */
 	bool		p_is_insert;	/* process assignment like INSERT not UPDATE */
 	List	   *p_windowdefs;	/* raw representations of window clauses */