SNOW-2028051 introduce a new client_fetch_threads connection parameter to decouple threads number limitations on fetching and pre-fetching (#2255)

Author: sfc-gh-mmishchenko

DESCRIPTION.md

@@ -22,6 +22,7 @@ Source code is also available at: https://github.com/snowflakedb/snowflake-conne
   - Lower log levels from info to debug for some of the messages to make the output easier to follow.
   - Allow the connector to inherit a UUID4 generated upstream, provided in statement parameters (field: `requestId`), rather than automatically generate a UUID4 to use for the HTTP Request ID.
   - Fix expired S3 credentials update and increment retry when expired credentials are found.
+  - Added `client_fetch_threads` experimental parameter to better utilize threads for fetching query results.
 
 - v3.14.0(March 03, 2025)
   - Bumped pyOpenSSL dependency upper boundary from <25.0.0 to <26.0.0.

src/snowflake/connector/connection.py

@@ -125,6 +125,7 @@
 
 DEFAULT_CLIENT_PREFETCH_THREADS = 4
 MAX_CLIENT_PREFETCH_THREADS = 10
+MAX_CLIENT_FETCH_THREADS = 1024
 DEFAULT_BACKOFF_POLICY = exponential_backoff()
 
 
@@ -228,6 +229,7 @@ def _get_private_bytes_from_file(
         (type(None), int),
     ),  # snowflake
     "client_prefetch_threads": (4, int),  # snowflake
+    "client_fetch_threads": (None, (type(None), int)),
     "numpy": (False, bool),  # snowflake
     "ocsp_response_cache_filename": (None, (type(None), str)),  # snowflake internal
     "converter_class": (DefaultConverterClass(), SnowflakeConverter),
@@ -417,6 +419,7 @@ class SnowflakeConnection:
             See the backoff_policies module for details and implementation examples.
         client_session_keep_alive_heartbeat_frequency: Heartbeat frequency to keep connection alive in seconds.
         client_prefetch_threads: Number of threads to download the result set.
+        client_fetch_threads: Number of threads to fetch staged query results.
         rest: Snowflake REST API object. Internal use only. Maybe removed in a later release.
         application: Application name to communicate with Snowflake as. By default, this is "PythonConnector".
         errorhandler: Handler used with errors. By default, an exception will be raised on error.
@@ -681,6 +684,16 @@ def client_prefetch_threads(self, value) -> None:
         self._client_prefetch_threads = value
         self._validate_client_prefetch_threads()
 
+    @property
+    def client_fetch_threads(self) -> int | None:
+        return self._client_fetch_threads
+
+    @client_fetch_threads.setter
+    def client_fetch_threads(self, value: None | int) -> None:
+        if value is not None:
+            value = min(max(1, value), MAX_CLIENT_FETCH_THREADS)
+        self._client_fetch_threads = value
+
     @property
     def rest(self) -> SnowflakeRestful | None:
         return self._rest

src/snowflake/connector/cursor.py

@@ -1208,7 +1208,8 @@ def _init_result_and_meta(self, data: dict[Any, Any]) -> None:
         self._result_set = ResultSet(
             self,
             result_chunks,
-            self._connection.client_prefetch_threads,
+            self._connection.client_fetch_threads
+            or self._connection.client_prefetch_threads,
         )
         self._rownumber = -1
         self._result_state = ResultState.VALID

test/integ/test_connection.py

@@ -1141,6 +1141,15 @@ def test_client_prefetch_threads_setting(conn_cnx):
         assert conn.client_prefetch_threads == new_thread_count
 
 
+@pytest.mark.skipolddriver
+def test_client_fetch_threads_setting(conn_cnx):
+    """Tests whether client_fetch_threads is None by default and setting the parameter has effect."""
+    with conn_cnx() as conn:
+        assert conn.client_fetch_threads is None
+        conn.client_fetch_threads = 32
+        assert conn.client_fetch_threads == 32
+
+
 @pytest.mark.external
 def test_client_failover_connection_url(conn_cnx):
     with conn_cnx("client_failover") as conn: