Add a table of features to the readme.

README.rst

@@ -67,6 +67,56 @@ currently supported. RelStorage replaced the PGStorage project.
 .. _zodbpack: https://relstorage.readthedocs.io/en/latest/zodbpack.html_
 .. _zodburi: https://relstorage.readthedocs.io/en/latest/zodburi.html
 
+Features Supported by Databases
+===============================
+
+Some of RelStorage's features are only supported on certain versions
+of certain databases. If the database doesn't support the feature,
+RelStorage will still work, but possibly with a performance penalty.
+
+
+.. list-table:: Supported Features
+   :widths: auto
+   :header-rows: 1
+   :stub-columns: 1
+
+   * -
+     - Parallel Commit
+     - Shared readCurrent locks
+     - Non-blocking readCurrent locks
+     - Streaming blobs
+     - Central transaction ID allocation
+     - Atomic lock and commit without Python involvement
+   * - PostgreSQL
+     - Yes
+     - Yes
+     - Yes
+     - With psycopg2 driver
+     - Yes
+     - Yes, except with PG8000 driver
+   * - MySQL
+     - Yes
+     - Yes
+     - Native on MySQL 8.0, emulated on MySQL 5.7
+     - No, emulated via chunking
+     - Yes
+     - Yes
+   * - Oracle
+     - Yes
+     - No
+     - Yes
+     - Yes
+     - No (could probably be implemented)
+     - No (could probably be implemented)
+   * - SQLite
+     - No
+     - No
+     - N/A (there is no distinction in lock types)
+     - No, consider using a shared-blob-dir
+     - N/A (essentially yes because it happens on one machine)
+     - No
+
+
 ===============
  Documentation
 ===============
@@ -93,7 +143,7 @@ RelStorage is hosted at GitHub:
     https://github.com/zodb/relstorage
 
 Continuous integration
-----------------------
+======================
 
 A test suite is run for every push and pull request submitted. Travis
 CI is used to test on Linux, and AppVeyor runs the builds on

src/relstorage/adapters/interfaces.py

@@ -965,6 +965,13 @@ def deleteObject(cursor, oid_int, tid_int):
 class IPoller(Interface):
     """Poll for new data"""
 
+    def get_current_tid(cursor):
+        """
+        Returns the highest transaction ID visible to the cursor.
+
+        If there are no transactions, returns 0.
+        """
+
     def poll_invalidations(conn, cursor, prev_polled_tid):
         """
         Polls for new transactions.

src/relstorage/adapters/sql/query.py

@@ -217,8 +217,6 @@ def execute(self, cursor, params=None):
                 cursor.execute(self._prepare_stmt)
             params = self._prepare_converter(params)
 
-        __traceback_info__ = stmt, params
-
         if params:
             cursor.execute(stmt, params)
         elif self.params:

src/relstorage/cache/mvcc.py

@@ -596,11 +596,10 @@ def __set_viewer_state_locked(self, cache, index):
                     if index is not None
                     else None)
 
-    def __initial_poll(self, cache, conn, cursor):
+    def __initial_poll(self, cache, cursor):
         # Initial poll for the world.
-        change_iter, tid = cache.adapter.poller.poll_invalidations(conn, cursor, None)
+        tid = cache.adapter.poller.get_current_tid(cursor)
 
-        assert change_iter is None
         new_index = None
         if tid > 0:
             # tid 0 is empty database, no data.
@@ -614,8 +613,7 @@ def __initial_poll(self, cache, conn, cursor):
             self.__set_viewer_state_locked(cache, self.object_index)
         # Regardless whether we or someone else did or did not
         # get an index, the viewer gets our current state, and also
-        # gets told to invalidate everything.
-        return change_iter
+        # gets told to invalidate everything (implicit return None)
 
     def _poll(self, cache, conn, cursor,
               current_index,
@@ -624,7 +622,7 @@ def _poll(self, cache, conn, cursor,
         # or it can return (None, old_tid) where old_tid is less than
         # its third parameter (``prev_polled_tid``)
         if current_index is None:
-            return self.__initial_poll(cache, conn, cursor)
+            return self.__initial_poll(cache, cursor)
 
         # We have begun keeping an object index. But the cache
         # may not yet. (See comments in _ObjectIndex docstring about

src/relstorage/cache/tests/test_mvcc.py

@@ -68,6 +68,10 @@ def setUp(self):
         self.polled_tid = 0
         self.polled_changes = None
         self.viewer.adapter.poller.poll_invalidations = self.poll_invalidations
+        self.viewer.adapter.poller.get_current_tid = self.get_current_tid
+
+    def get_current_tid(self, _cursor):
+        return self.polled_tid
 
     def poll_invalidations(self, conn, cursor, last_tid):
         self.assertEqual(last_tid, self.expected_poll_last_tid)