fix: dry run queries with DB API cursor

google/cloud/bigquery/dbapi/cursor.py

@@ -15,6 +15,7 @@
 """Cursor for the Google BigQuery DB-API."""
 
 import collections
+import copy
 
 try:
     from collections import abc as collections_abc
@@ -26,6 +27,7 @@
 import six
 
 from google.cloud.bigquery import job
+from google.cloud.bigquery import table
 from google.cloud.bigquery.dbapi import _helpers
 from google.cloud.bigquery.dbapi import exceptions
 import google.cloud.exceptions
@@ -169,12 +171,24 @@ def execute(self, operation, parameters=None, job_id=None, job_config=None):
         formatted_operation = _format_operation(operation, parameters=parameters)
         query_parameters = _helpers.to_query_parameters(parameters)
 
-        config = job_config or job.QueryJobConfig(use_legacy_sql=False)
+        if client._default_query_job_config:
+            if job_config:
+                config = job_config._fill_from_default(client._default_query_job_config)
+            else:
+                config = copy.deepcopy(client._default_query_job_config)
+        else:
+            config = job_config or job.QueryJobConfig(use_legacy_sql=False)
+
         config.query_parameters = query_parameters
         self._query_job = client.query(
             formatted_operation, job_config=config, job_id=job_id
         )
 
+        if self._query_job.dry_run:
+            self.rowcount = 1
+            self.description = None
+            return
+
         # Wait for the query to finish.
         try:
             self._query_job.result()
@@ -207,6 +221,12 @@ def _try_fetch(self, size=None):
                 "No query results: execute() must be called before fetch."
             )
 
+        if self._query_job.dry_run:
+            estimated_bytes = self._query_job.total_bytes_processed
+            row = table.Row((estimated_bytes,), {"estimated_bytes": 0})
+            self._query_data = iter([row])
+            return
+
         is_dml = (
             self._query_job.statement_type
             and self._query_job.statement_type.upper() != "SELECT"
@@ -290,6 +310,11 @@ def _bqstorage_fetch(self, bqstorage_client):
     def fetchone(self):
         """Fetch a single row from the results of the last ``execute*()`` call.
 
+        .. note::
+            If a dry run query was executed, a row with a single value is
+            returned representing the estimated number of bytes that would be
+            processed by the query.
+
         Returns:
             Tuple:
                 A tuple representing a row or ``None`` if no more data is
@@ -307,6 +332,11 @@ def fetchone(self):
     def fetchmany(self, size=None):
         """Fetch multiple results from the last ``execute*()`` call.
 
+        .. note::
+            If a dry run query was executed, a row with a single value is
+            returned representing the estimated number of bytes that would be
+            processed by the query.
+
         .. note::
             The size parameter is not used for the request/response size.
             Set the ``arraysize`` attribute before calling ``execute()`` to
@@ -343,6 +373,11 @@ def fetchmany(self, size=None):
     def fetchall(self):
         """Fetch all remaining results from the last ``execute*()`` call.
 
+        .. note::
+            If a dry run query was executed, a row with a single value is
+            returned representing the estimated number of bytes that would be
+            processed by the query.
+
         Returns:
             List[Tuple]: A list of all the rows in the results.
 

tests/system.py

@@ -1782,6 +1782,24 @@ def test_dbapi_fetch_w_bqstorage_client_large_result_set(self):
         ]
         self.assertEqual(fetched_data, expected_data)
 
+    def test_dbapi_dry_run_query(self):
+        from google.cloud.bigquery.job import QueryJobConfig
+
+        query = """
+            SELECT country_name
+            FROM `bigquery-public-data.utility_us.country_code_iso`
+            WHERE country_name LIKE 'U%'
+        """
+
+        Config.CURSOR.execute(query, job_config=QueryJobConfig(dry_run=True))
+        self.assertEqual(Config.CURSOR.rowcount, 1, "expected a single row")
+
+        rows = Config.CURSOR.fetchall()
+
+        row_tuples = [r.values() for r in rows]
+        expected = [(3473,)]
+        self.assertEqual(row_tuples, expected)
+
     def _load_table_for_dml(self, rows, dataset_id, table_id):
         from google.cloud._testing import _NamedTemporaryFile
         from google.cloud.bigquery.job import CreateDisposition

tests/unit/test_dbapi_cursor.py

@@ -36,7 +36,15 @@ def _get_target_class():
     def _make_one(self, *args, **kw):
         return self._get_target_class()(*args, **kw)
 
-    def _mock_client(self, rows=None, schema=None, num_dml_affected_rows=None):
+    def _mock_client(
+        self,
+        rows=None,
+        schema=None,
+        num_dml_affected_rows=None,
+        default_query_job_config=None,
+        dry_run_job=False,
+        total_bytes_processed=0,
+    ):
         from google.cloud.bigquery import client
 
         if rows is None:
@@ -49,8 +57,12 @@ def _mock_client(self, rows=None, schema=None, num_dml_affected_rows=None):
             total_rows=total_rows,
             schema=schema,
             num_dml_affected_rows=num_dml_affected_rows,
+            dry_run=dry_run_job,
+            total_bytes_processed=total_bytes_processed,
         )
         mock_client.list_rows.return_value = rows
+        mock_client._default_query_job_config = default_query_job_config
+
         return mock_client
 
     def _mock_bqstorage_client(self, rows=None, stream_count=0):
@@ -76,18 +88,31 @@ def _mock_bqstorage_client(self, rows=None, stream_count=0):
 
         return mock_client
 
-    def _mock_job(self, total_rows=0, schema=None, num_dml_affected_rows=None):
+    def _mock_job(
+        self,
+        total_rows=0,
+        schema=None,
+        num_dml_affected_rows=None,
+        dry_run=False,
+        total_bytes_processed=0,
+    ):
         from google.cloud.bigquery import job
 
         mock_job = mock.create_autospec(job.QueryJob)
         mock_job.error_result = None
         mock_job.state = "DONE"
-        mock_job.result.return_value = mock_job
-        mock_job._query_results = self._mock_results(
-            total_rows=total_rows,
-            schema=schema,
-            num_dml_affected_rows=num_dml_affected_rows,
-        )
+        mock_job.dry_run = dry_run
+
+        if dry_run:
+            mock_job.result.side_effect = exceptions.NotFound
+            mock_job.total_bytes_processed = total_bytes_processed
+        else:
+            mock_job.result.return_value = mock_job
+            mock_job._query_results = self._mock_results(
+                total_rows=total_rows,
+                schema=schema,
+                num_dml_affected_rows=num_dml_affected_rows,
+            )
 
         if num_dml_affected_rows is None:
             mock_job.statement_type = None  # API sends back None for SELECT
@@ -373,7 +398,27 @@ def test_execute_custom_job_id(self):
         self.assertEqual(args[0], "SELECT 1;")
         self.assertEqual(kwargs["job_id"], "foo")
 
-    def test_execute_custom_job_config(self):
+    def test_execute_w_default_config(self):
+        from google.cloud.bigquery.dbapi import connect
+        from google.cloud.bigquery import job
+
+        default_config = job.QueryJobConfig(use_legacy_sql=False, flatten_results=True)
+        client = self._mock_client(
+            rows=[], num_dml_affected_rows=0, default_query_job_config=default_config
+        )
+        connection = connect(client)
+        cursor = connection.cursor()
+
+        cursor.execute("SELECT 1;", job_id="foo")
+
+        _, kwargs = client.query.call_args
+        used_config = kwargs["job_config"]
+        expected_config = job.QueryJobConfig(
+            use_legacy_sql=False, flatten_results=True, query_parameters=[]
+        )
+        self.assertEqual(used_config._properties, expected_config._properties)
+
+    def test_execute_custom_job_config_wo_default_config(self):
         from google.cloud.bigquery.dbapi import connect
         from google.cloud.bigquery import job
 
@@ -387,6 +432,29 @@ def test_execute_custom_job_config(self):
         self.assertEqual(kwargs["job_id"], "foo")
         self.assertEqual(kwargs["job_config"], config)
 
+    def test_execute_custom_job_config_w_default_config(self):
+        from google.cloud.bigquery.dbapi import connect
+        from google.cloud.bigquery import job
+
+        default_config = job.QueryJobConfig(use_legacy_sql=False, flatten_results=True)
+        client = self._mock_client(
+            rows=[], num_dml_affected_rows=0, default_query_job_config=default_config
+        )
+        connection = connect(client)
+        cursor = connection.cursor()
+        config = job.QueryJobConfig(use_legacy_sql=True)
+
+        cursor.execute("SELECT 1;", job_id="foo", job_config=config)
+
+        _, kwargs = client.query.call_args
+        used_config = kwargs["job_config"]
+        expected_config = job.QueryJobConfig(
+            use_legacy_sql=True,  # the config passed to execute() prevails
+            flatten_results=True,  # from the default
+            query_parameters=[],
+        )
+        self.assertEqual(used_config._properties, expected_config._properties)
+
     def test_execute_w_dml(self):
         from google.cloud.bigquery.dbapi import connect
 
@@ -442,6 +510,40 @@ def test_execute_w_query(self):
         row = cursor.fetchone()
         self.assertIsNone(row)
 
+    def test_execute_w_query_dry_run(self):
+        from google.cloud.bigquery.job import QueryJobConfig
+        from google.cloud.bigquery.schema import SchemaField
+        from google.cloud.bigquery import dbapi
+
+        connection = dbapi.connect(
+            self._mock_client(
+                rows=[("hello", "world", 1), ("howdy", "y'all", 2)],
+                schema=[
+                    SchemaField("a", "STRING", mode="NULLABLE"),
+                    SchemaField("b", "STRING", mode="REQUIRED"),
+                    SchemaField("c", "INTEGER", mode="NULLABLE"),
+                ],
+                dry_run_job=True,
+                total_bytes_processed=12345,
+            )
+        )
+        cursor = connection.cursor()
+        cursor.execute(
+            "SELECT a, b, c FROM hello_world WHERE d > 3;",
+            job_config=QueryJobConfig(dry_run=True),
+        )
+
+        self.assertIsNone(cursor.description)
+        self.assertEqual(cursor.rowcount, 1)
+
+        rows = cursor.fetchall()
+
+        # We expect a single row with one column - the estimated numbe of bytes
+        # that will be processed by the query.
+        self.assertEqual(len(rows), 1)
+        self.assertEqual(len(rows[0]), 1)
+        self.assertEqual(rows[0][0], 12345)
+
     def test_execute_raises_if_result_raises(self):
         import google.cloud.exceptions
 
@@ -451,8 +553,10 @@ def test_execute_raises_if_result_raises(self):
         from google.cloud.bigquery.dbapi import exceptions
 
         job = mock.create_autospec(job.QueryJob)
+        job.dry_run = None
         job.result.side_effect = google.cloud.exceptions.GoogleCloudError("")
         client = mock.create_autospec(client.Client)
+        client._default_query_job_config = None
         client.query.return_value = job
         connection = connect(client)
         cursor = connection.cursor()