test: add dedicated test for empty struct array unnest schema

Author: shuoweil

tests/system/small/test_multiindex.py

@@ -1490,28 +1490,3 @@ def test_multiindex_eq_const(scalars_df_index, scalars_pandas_df_index):
     bigframes.testing.utils.assert_index_equal(
         pandas.Index(pd_result, dtype="boolean"), bf_result.to_pandas()
     )
-
-
-def test_count_empty_multiindex_columns(session):
-    df = pandas.DataFrame(
-        [], index=[1, 2], columns=pandas.MultiIndex.from_tuples([], names=["a", "b"])
-    )
-    bdf = session.read_pandas(df)
-
-    # count() operation unpivots columns, triggering the empty MultiIndex bug internally
-    count_df = bdf.count().to_pandas()
-    assert count_df.shape == (0,)
-    assert count_df.index.nlevels == 2
-    assert list(count_df.index.names) == ["a", "b"]
-
-
-def test_count_preserves_multiindex_columns(session):
-    # Tests that `melt` operations via count do not cause MultiIndex drops in Arrow
-    df = pandas.DataFrame({"A": [1], "B": ["string"], "C": [3]})
-    df.columns = pandas.MultiIndex.from_tuples(
-        [("Group1", "A"), ("Group2", "B"), ("Group1", "C")]
-    )
-    bdf = session.read_pandas(df)
-
-    count_df = bdf.count().to_pandas()
-    assert count_df.shape[0] == 3

tests/unit/core/compile/sqlglot/test_compile_readlocal.py

@@ -14,9 +14,13 @@
 
 import numpy as np
 import pandas as pd
+import pyarrow as pa
 import pytest
 
 import bigframes
+from bigframes.core import schema
+from bigframes.core.compile.sqlglot.sqlglot_ir import SQLGlotIR
+from bigframes.core.guid import SequentialUIDGenerator
 import bigframes.pandas as bpd
 
 pytest.importorskip("pytest_snapshot")
@@ -76,3 +80,12 @@ def test_compile_readlocal_w_special_values(
     )
     bf_df = bpd.DataFrame(df, session=compiler_session)
     snapshot.assert_match(bf_df.sql, "out.sql")
+
+
+def test_compile_from_pyarrow_empty_schema():
+    pa_table = pa.Table.from_arrays([], names=[])
+    schema_obj = schema.ArraySchema(items=())
+    uid_gen = SequentialUIDGenerator()
+    ir = SQLGlotIR.from_pyarrow(pa_table, schema_obj, uid_gen)
+    sql_str = ir.sql
+    assert "__empty_struct_dummy__" in sql_str