1 files changed, 15 insertions, 18 deletions
diff --git a/sqlglot/transforms.py b/sqlglot/transforms.py
index 35ff75a..aa7d240 100644
--- a/sqlglot/transforms.py
+++ b/sqlglot/transforms.py
@@ -27,20 +27,18 @@ def unalias_group(expression: exp.Expression) -> exp.Expression:
     """
     if isinstance(expression, exp.Group) and isinstance(expression.parent, exp.Select):
         aliased_selects = {
-            e.alias: (i, e.this)
+            e.alias: i
             for i, e in enumerate(expression.parent.expressions, start=1)
             if isinstance(e, exp.Alias)
         }
 
-        expression = expression.copy()
-
-        top_level_expression = None
-        for item, parent, _ in expression.walk(bfs=False):
-            top_level_expression = item if isinstance(parent, exp.Group) else top_level_expression
-            if isinstance(item, exp.Column) and not item.table:
-                alias_index, col_expression = aliased_selects.get(item.name, (None, None))
-                if alias_index and top_level_expression != col_expression:
-                    item.replace(exp.Literal.number(alias_index))
+        for group_by in expression.expressions:
+            if (
+                isinstance(group_by, exp.Column)
+                and not group_by.table
+                and group_by.name in aliased_selects
+            ):
+                group_by.replace(exp.Literal.number(aliased_selects.get(group_by.name)))
 
     return expression
 
@@ -63,22 +61,21 @@ def eliminate_distinct_on(expression: exp.Expression) -> exp.Expression:
         and expression.args["distinct"].args.get("on")
         and isinstance(expression.args["distinct"].args["on"], exp.Tuple)
     ):
-        distinct_cols = [e.copy() for e in expression.args["distinct"].args["on"].expressions]
-        outer_selects = [e.copy() for e in expression.expressions]
-        nested = expression.copy()
-        nested.args["distinct"].pop()
+        distinct_cols = expression.args["distinct"].args["on"].expressions
+        expression.args["distinct"].pop()
+        outer_selects = expression.selects
         row_number = find_new_name(expression.named_selects, "_row_number")
         window = exp.Window(
             this=exp.RowNumber(),
             partition_by=distinct_cols,
         )
-        order = nested.args.get("order")
+        order = expression.args.get("order")
         if order:
             window.set("order", order.copy())
             order.pop()
         window = exp.alias_(window, row_number)
-        nested.select(window, copy=False)
-        return exp.select(*outer_selects).from_(nested.subquery()).where(f'"{row_number}" = 1')
+        expression.select(window, copy=False)
+        return exp.select(*outer_selects).from_(expression.subquery()).where(f'"{row_number}" = 1')
     return expression
 
 
@@ -120,7 +117,7 @@ def preprocess(
     """
 
     def _to_sql(self, expression):
-        expression = transforms[0](expression)
+        expression = transforms[0](expression.copy())
         for t in transforms[1:]:
             expression = t(expression)
         return to_sql(self, expression)