Revert "Fix(optimizer)!: parse unqualified names in DISTINCT ON (...) as iden…" (#4804)

This reverts commit 14c3de9.

Author: georgesittas

sqlglot/parser.py

@@ -3089,21 +3089,10 @@ def _parse_select(
             )
 
             if distinct:
-                on = self._parse_value() if self._match(TokenType.ON) else None
-
-                # In `SELECT DISTINCT ON (p1, t.c2)`, p1 is an identifier, while c2 is a column reference.
-                # The reason is that `p1` can be a projection alias or a column, but engines resolve it as
-                # the projection alias (e.g. postgres, duckdb, clickhouse).
-                if isinstance(on, exp.Tuple):
-                    on.set(
-                        "expressions",
-                        [
-                            c.this if isinstance(c, exp.Column) and not c.table else c
-                            for c in on.expressions
-                        ],
-                    )
-
-                distinct = self.expression(exp.Distinct, on=on)
+                distinct = self.expression(
+                    exp.Distinct,
+                    on=self._parse_value() if self._match(TokenType.ON) else None,
+                )
 
             if all_ and distinct:
                 self.raise_error("Cannot specify both ALL and DISTINCT after SELECT")

tests/fixtures/optimizer/qualify_columns.sql

@@ -90,14 +90,6 @@ SELECT x.a AS a, x.b AS b FROM x AS x ORDER BY a, b;
 SELECT DISTINCT a AS c, b AS d FROM x ORDER BY 1;
 SELECT DISTINCT x.a AS c, x.b AS d FROM x AS x ORDER BY c;
 
-# execute: false
-SELECT DISTINCT ON (x, t.y) * FROM t;
-SELECT DISTINCT ON (x, t.y) * FROM t AS t;
-
-# execute: false
-SELECT DISTINCT ON (new_col) t1.col1 AS new_col FROM table1 AS t1;
-SELECT DISTINCT ON (new_col) t1.col1 AS new_col FROM table1 AS t1;
-
 SELECT 2 FROM x GROUP BY 1;
 SELECT 2 AS "2" FROM x AS x GROUP BY 1;