Use fast path in plpgsql's RETURN/RETURN NEXT in more cases.

exec_stmt_return() and exec_stmt_return_next() have fast-path code for
handling a simple variable reference (i.e. "return var") without going
through the full expression evaluation machinery.  For some reason,
pl_gram.y was under the impression that this fast path only applied for
record/row variables; but in reality code for handling regular scalar
variables has been there all along.  Adjusting the logic to allow that
code to be used actually results in a net savings of code in pl_gram.y
(by eliminating some redundancy), and it buys a measurable though not
very impressive amount of speedup.

Noted while fooling with my expanded-array patch, wherein this makes a much
bigger difference because it enables returning an expanded array variable
without an extra flattening step.  But AFAICS this is a win regardless,
so commit it separately.

src/pl/plpgsql/src/pl_exec.c

@@ -2452,8 +2452,9 @@ exec_stmt_return(PLpgSQL_execstate *estate, PLpgSQL_stmt_return *stmt)
 	estate->retisnull = true;
 
 	/*
-	 * This special-case path covers record/row variables in fn_retistuple
-	 * functions, as well as functions with one or more OUT parameters.
+	 * Special case path when the RETURN expression is a simple variable
+	 * reference; in particular, this path is always taken in functions with
+	 * one or more OUT parameters.
 	 */
 	if (stmt->retvarno >= 0)
 	{
@@ -2576,8 +2577,9 @@ exec_stmt_return_next(PLpgSQL_execstate *estate,
 	natts = tupdesc->natts;
 
 	/*
-	 * This special-case path covers record/row variables in fn_retistuple
-	 * functions, as well as functions with one or more OUT parameters.
+	 * Special case path when the RETURN NEXT expression is a simple variable
+	 * reference; in particular, this path is always taken in functions with
+	 * one or more OUT parameters.
 	 */
 	if (stmt->retvarno >= 0)
 	{

src/pl/plpgsql/src/pl_gram.y

@@ -3036,16 +3036,17 @@ make_return_stmt(int location)
 					 errmsg("RETURN cannot have a parameter in function returning void"),
 					 parser_errposition(yylloc)));
 	}
-	else if (plpgsql_curr_compile->fn_retistuple)
+	else
 	{
 		/*
-		 * We want to special-case simple row or record references for
-		 * efficiency.  So peek ahead to see if that's what we have.
+		 * We want to special-case simple variable references for efficiency.
+		 * So peek ahead to see if that's what we have.
 		 */
 		int		tok = yylex();
 
 		if (tok == T_DATUM && plpgsql_peek() == ';' &&
-			(yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_ROW ||
+			(yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_VAR ||
+			 yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_ROW ||
 			 yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_REC))
 		{
 			new->retvarno = yylval.wdatum.datum->dno;
@@ -3055,19 +3056,16 @@ make_return_stmt(int location)
 		}
 		else
 		{
-			/* Not (just) a row/record name, so treat as expression */
+			/*
+			 * Not (just) a variable name, so treat as expression.
+			 *
+			 * Note that a well-formed expression is _required_ here;
+			 * anything else is a compile-time error.
+			 */
 			plpgsql_push_back_token(tok);
 			new->expr = read_sql_expression(';', ";");
 		}
 	}
-	else
-	{
-		/*
-		 * Note that a well-formed expression is _required_ here;
-		 * anything else is a compile-time error.
-		 */
-		new->expr = read_sql_expression(';', ";");
-	}
 
 	return (PLpgSQL_stmt *) new;
 }
@@ -3099,16 +3097,17 @@ make_return_next_stmt(int location)
 					 parser_errposition(yylloc)));
 		new->retvarno = plpgsql_curr_compile->out_param_varno;
 	}
-	else if (plpgsql_curr_compile->fn_retistuple)
+	else
 	{
 		/*
-		 * We want to special-case simple row or record references for
-		 * efficiency.  So peek ahead to see if that's what we have.
+		 * We want to special-case simple variable references for efficiency.
+		 * So peek ahead to see if that's what we have.
 		 */
 		int		tok = yylex();
 
 		if (tok == T_DATUM && plpgsql_peek() == ';' &&
-			(yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_ROW ||
+			(yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_VAR ||
+			 yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_ROW ||
 			 yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_REC))
 		{
 			new->retvarno = yylval.wdatum.datum->dno;
@@ -3118,13 +3117,16 @@ make_return_next_stmt(int location)
 		}
 		else
 		{
-			/* Not (just) a row/record name, so treat as expression */
+			/*
+			 * Not (just) a variable name, so treat as expression.
+			 *
+			 * Note that a well-formed expression is _required_ here;
+			 * anything else is a compile-time error.
+			 */
 			plpgsql_push_back_token(tok);
 			new->expr = read_sql_expression(';', ";");
 		}
 	}
-	else
-		new->expr = read_sql_expression(';', ";");
 
 	return (PLpgSQL_stmt *) new;
 }