fix(pandas): make case work for non-RangeIndex dataframes (#9083)

## Description of changes

This PR makes `PandasExecutor` create the `Series` with an index that
matches the incoming data. Currently, when the incoming data does not
use a `RangeIndex`, the output index is a union of a `RangeIndex` and
the incoming data index.

ibis/backends/pandas/executor.py

@@ -170,9 +170,10 @@ def visit(
         if base is not None:
             cases = tuple(base == case for case in cases)
         cases, _ = cls.asframe(cases, concat=False)
+        index = cases[0].index
         results, _ = cls.asframe(results, concat=False)
         out = np.select(cases, results, default)
-        return pd.Series(out)
+        return pd.Series(out, index=index)
 
     @classmethod
     def visit(cls, op: ops.TimestampTruncate | ops.DateTruncate, arg, unit):

ibis/backends/pandas/tests/test_operations.py

@@ -741,6 +741,30 @@ def test_simple_case_column(batting, batting_df):
     tm.assert_series_equal(result, expected)
 
 
+def test_non_range_index():
+    def do_replace(col):
+        return col.cases(
+            (
+                (1, "one"),
+                (2, "two"),
+            ),
+            default="unk",
+        )
+
+    df = pd.DataFrame(
+        {
+            "A": pd.Series({i: i % 3 for i in (0, 1, 2, 4)}),
+            "B": 0,
+        }
+    )
+    expr = (
+        ibis.pandas.connect({"t": df})
+        .table("t")
+        .mutate(A=lambda t: t["A"].pipe(do_replace))
+    )
+    assert df.index.equals(expr.execute().index)
+
+
 def test_table_distinct(t, df):
     expr = t[["dup_strings"]].distinct()
     result = expr.execute()