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/pl/plpgsql/src/pl_exec.c

@@ -3995,7 +3995,7 @@ 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/MERGE
+	 * whether the statement is INSERT/UPDATE/DELETE
 	 */
 	if (expr->plan == NULL)
 	{
@@ -4016,7 +4016,6 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 				{
 					if (q->commandType == CMD_INSERT ||
 						q->commandType == CMD_UPDATE ||
-						q->commandType == CMD_MERGE ||
 						q->commandType == CMD_DELETE)
 						stmt->mod_stmt = true;
 				}
@@ -4074,7 +4073,6 @@ 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;
@@ -4252,7 +4250,6 @@ 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;

src/pl/plpgsql/src/pl_gram.y

@@ -304,7 +304,6 @@ 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
@@ -1952,10 +1951,6 @@ stmt_execsql	: K_IMPORT
 					{
 						$$ = make_execsql_stmt(K_INSERT, @1);
 					}
-				| K_MERGE
-					{
-						$$ = make_execsql_stmt(K_MERGE, @1);
-					}
 				| T_WORD
 					{
 						int			tok;
@@ -2502,7 +2497,6 @@ unreserved_keyword	:
 				| K_IS
 				| K_LAST
 				| K_LOG
-				| K_MERGE
 				| K_MESSAGE
 				| K_MESSAGE_TEXT
 				| K_MOVE
@@ -2966,8 +2960,6 @@ 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)

src/pl/plpgsql/src/pl_scanner.c

@@ -138,7 +138,6 @@ static const ScanKeyword unreserved_keywords[] = {
 	PG_KEYWORD("is", K_IS, UNRESERVED_KEYWORD)
 	PG_KEYWORD("last", K_LAST, UNRESERVED_KEYWORD)
 	PG_KEYWORD("log", K_LOG, UNRESERVED_KEYWORD)
-	PG_KEYWORD("merge", K_MERGE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("message", K_MESSAGE, UNRESERVED_KEYWORD)
 	PG_KEYWORD("message_text", K_MESSAGE_TEXT, UNRESERVED_KEYWORD)
 	PG_KEYWORD("move", K_MOVE, UNRESERVED_KEYWORD)

src/pl/plpgsql/src/plpgsql.h

@@ -857,8 +857,8 @@ typedef struct PLpgSQL_stmt_execsql
 	PLpgSQL_stmt_type cmd_type;
 	int			lineno;
 	PLpgSQL_expr *sqlstmt;
-	bool		mod_stmt;		/* is the stmt INSERT/UPDATE/DELETE/MERGE?
-								 * Note mod_stmt is set when we plan the query */
+	bool		mod_stmt;		/* is the stmt INSERT/UPDATE/DELETE?  Note:
+								 * mod_stmt is set when we plan the query */
 	bool		into;			/* INTO supplied? */
 	bool		strict;			/* INTO STRICT flag */
 	PLpgSQL_variable *target;	/* INTO target (record or row) */