Allow empty target list in SELECT.

This fixes a problem noted as a followup to bug #8648: if a query has a
semantically-empty target list, e.g. SELECT * FROM zero_column_table,
ruleutils.c will dump it as a syntactically-empty target list, which was
not allowed.  There doesn't seem to be any reliable way to fix this by
hacking ruleutils (note in particular that the originally zero-column table
might since have had columns added to it); and even if we had such a fix,
it would do nothing for existing dump files that might contain bad syntax.
The best bet seems to be to relax the syntactic restriction.

Also, add parse-analysis errors for SELECT DISTINCT with no columns (after
*-expansion) and RETURNING with no columns.  These cases previously
produced unexpected behavior because the parsed Query looked like it had
no DISTINCT or RETURNING clause, respectively.  If anyone ever offers
a plausible use-case for this, we could work a bit harder on making the
situation distinguishable.

Arguably this is a bug fix that should be back-patched, but I'm worried
that there may be client apps or PLs that expect "SELECT ;" to throw a
syntax error.  The issue doesn't seem important enough to risk changing
behavior in minor releases.

src/backend/parser/analyze.c

@@ -2018,6 +2018,19 @@ transformReturningList(ParseState *pstate, List *returningList)
 	/* transform RETURNING identically to a SELECT targetlist */
 	rlist = transformTargetList(pstate, returningList, EXPR_KIND_RETURNING);
 
+	/*
+	 * Complain if the nonempty tlist expanded to nothing (which is possible
+	 * if it contains only a star-expansion of a zero-column table).  If we
+	 * allow this, the parsed Query will look like it didn't have RETURNING,
+	 * with results that would probably surprise the user.
+	 */
+	if (rlist == NIL)
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 errmsg("RETURNING must have at least one column"),
+				 parser_errposition(pstate,
+									exprLocation(linitial(returningList)))));
+
 	/* mark column origins */
 	markTargetListOrigins(pstate, rlist);
 

src/backend/parser/gram.y

@@ -334,7 +334,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 				name_list from_clause from_list opt_array_bounds
 				qualified_name_list any_name any_name_list
 				any_operator expr_list attrs
-				target_list insert_column_list set_target_list
+				target_list opt_target_list insert_column_list set_target_list
 				set_clause_list set_clause multiple_set_clause
 				ctext_expr_list ctext_row def_list indirection opt_indirection
 				reloption_list group_clause TriggerFuncArgs select_limit
@@ -9259,7 +9259,7 @@ select_clause:
  * However, this is not checked by the grammar; parse analysis must check it.
  */
 simple_select:
-			SELECT opt_distinct target_list
+			SELECT opt_distinct opt_target_list
 			into_clause from_clause where_clause
 			group_clause having_clause window_clause
 				{
@@ -12215,6 +12215,10 @@ ctext_row: '(' ctext_expr_list ')'					{ $$ = $2; }
  *
  *****************************************************************************/
 
+opt_target_list: target_list						{ $$ = $1; }
+			| /* EMPTY */							{ $$ = NIL; }
+		;
+
 target_list:
 			target_el								{ $$ = list_make1($1); }
 			| target_list ',' target_el				{ $$ = lappend($1, $3); }

src/backend/parser/parse_clause.c

@@ -2011,6 +2011,20 @@ transformDistinctClause(ParseState *pstate,
 									  true);
 	}
 
+	/*
+	 * Complain if we found nothing to make DISTINCT.  Returning an empty list
+	 * would cause the parsed Query to look like it didn't have DISTINCT, with
+	 * results that would probably surprise the user.  Note: this case is
+	 * presently impossible for aggregates because of grammar restrictions,
+	 * but we check anyway.
+	 */
+	if (result == NIL)
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 is_agg ?
+				 errmsg("an aggregate with DISTINCT must have at least one argument") :
+				 errmsg("SELECT DISTINCT must have at least one column")));
+
 	return result;
 }
 
@@ -2115,6 +2129,11 @@ transformDistinctOnClause(ParseState *pstate, List *distinctlist,
 									  true);
 	}
 
+	/*
+	 * An empty result list is impossible here because of grammar restrictions.
+	 */
+	Assert(result != NIL);
+
 	return result;
 }