fix: Use actual BigQuery types rather than ibis types in to_pandas (#500)

* fix: use actual bigframes types rather than ibis types in to_pandas

* Use ibis function that properly converts nested bq datatypes to ibis type

Author: TrevorBergeron

bigframes/dtypes.py

@@ -682,6 +682,17 @@ def ibis_type_from_python_type(t: type) -> ibis_dtypes.DataType:
 
 
 def ibis_type_from_type_kind(tk: bigquery.StandardSqlTypeNames) -> ibis_dtypes.DataType:
+    """Convert bq type to ibis. Only to be used for remote functions, does not handle all types."""
     if tk not in SUPPORTED_IO_BIGQUERY_TYPEKINDS:
         raise UnsupportedTypeError(tk, SUPPORTED_IO_BIGQUERY_TYPEKINDS)
     return third_party_ibis_bqtypes.BigQueryType.to_ibis(tk)
+
+
+def bf_type_from_type_kind(bf_schema) -> Dict[str, Dtype]:
+    """Converts bigquery sql type to the default bigframes dtype."""
+    ibis_schema: ibis.Schema = third_party_ibis_bqtypes.BigQuerySchema.to_ibis(
+        bf_schema
+    )
+    return {
+        name: ibis_dtype_to_bigframes_dtype(type) for name, type in ibis_schema.items()
+    }

bigframes/session/__init__.py

@@ -1889,8 +1889,10 @@ def _get_table_size(self, destination_table):
     def _rows_to_dataframe(
         self, row_iterator: bigquery.table.RowIterator, dtypes: Dict
     ) -> pandas.DataFrame:
+        # Can ignore inferred datatype until dtype emulation breaks 1:1 mapping between BQ types and bigframes types
+        dtypes_from_bq = bigframes.dtypes.bf_type_from_type_kind(row_iterator.schema)
         arrow_table = row_iterator.to_arrow()
-        return bigframes.session._io.pandas.arrow_to_pandas(arrow_table, dtypes)
+        return bigframes.session._io.pandas.arrow_to_pandas(arrow_table, dtypes_from_bq)
 
     def _start_generic_job(self, job: formatting_helpers.GenericJob):
         if bigframes.options.display.progress_bar is not None: