perf: Add warning when using synthetic ordering on large table.

bigframes/exceptions.py

@@ -95,6 +95,10 @@ class ObsoleteVersionWarning(Warning):
     """The BigFrames version is too old."""
 
 
+class SyntheticTotalOrderWarning(Warning):
+    """Generating an expensive synthetic total ordering."""
+
+
 def format_message(message: str, fill: bool = True):
     """Formats a warning message with ANSI color codes for the warning color.
 

bigframes/session/loader.py

@@ -21,6 +21,7 @@
 import os
 import typing
 from typing import Dict, Hashable, IO, Iterable, List, Optional, Sequence, Tuple, Union
+import warnings
 
 import bigframes_vendored.constants as constants
 import bigframes_vendored.pandas.io.gbq as third_party_pandas_gbq
@@ -449,6 +450,20 @@ def read_gbq_table(
         # if we don't have a unique index, we order by row hash if we are in strict mode
         if self._force_total_order:
             if not primary_key:
+                # 5gb chosen rather arbitrarily, mostly want to avoid warning for very small tables, for which the
+                # price is trivial
+                if (table.num_bytes or 0) > 5000000000:
+                    msg = bigframes.exceptions.format_message(
+                        "Large table {table} has no natural ordering. "
+                        "Will use a synthetic natural ordering, this is very expensive. "
+                        "For better performance, use bigframes.option.bigquery.ordering_mode = 'partial' "
+                        "provide unique key as index_col argument, or set a valid primary key constraints on the table"
+                        "before importing it to a bigquery dataframe. "
+                        "(see: https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#primary_keys_and_foreign_keys).",
+                    )
+                    warnings.warn(
+                        msg, category=bigframes.exceptions.SyntheticTotalOrderWarning
+                    )
                 array_value = array_value.order_by(
                     [
                         bigframes.core.ordering.OrderingExpression(

tests/system/small/test_session.py

@@ -34,6 +34,7 @@
 import bigframes.core.indexes.base
 import bigframes.dataframe
 import bigframes.dtypes
+import bigframes.exceptions
 import bigframes.ml.linear_model
 from tests.system import utils
 
@@ -1557,3 +1558,25 @@ def test_read_json_gcs_default_engine(session, scalars_dfs, gcs_folder):
 
     assert df.shape[0] == scalars_df.shape[0]
     pd.testing.assert_series_equal(df.dtypes, scalars_df.dtypes)
+
+
+def test_read_gbq_large_table_ordering_warning(
+    session: bigframes.Session,
+):
+    with pytest.warns(
+        bigframes.exceptions.SyntheticTotalOrderWarning,
+    ):
+        # table is >10gb and has no primary key
+        session.read_gbq("bigquery-public-data.covid19_open_data.covid19_open_data")
+
+
+def test_read_gbq_large_table_no_ordering_no_warning(
+    unordered_session: bigframes.Session,
+):
+    with warnings.catch_warnings(
+        category=bigframes.exceptions.SyntheticTotalOrderWarning,
+    ):
+        # table is >10gb and has no primary key
+        unordered_session.read_gbq(
+            "bigquery-public-data.covid19_open_data.covid19_open_data"
+        )