(improvement) metadata: remove __dict__ from __slots__ for real memory savings, make _RowView a Mapping

- Remove '__dict__' and '__weakref__' from __slots__ on all metadata classes.
  All dynamic attribute assignments target attributes already declared in
  __slots__, so this is safe and gives real per-instance savings (~264 bytes
  per ColumnMetadata, ~516 KB for 2000-column schemas).

- Make _RowView inherit from collections.abc.Mapping and add __iter__/__len__,
  giving it a complete Mapping interface (keys/values/items for free).

- Add maintainability comment to _SELECT_COLUMNS noting coupling with
  _build_column_metadata.

Author: mykaul

cassandra/metadata.py

@@ -52,11 +52,15 @@
 from cassandra.util import maybe_add_timeout_to_query
 
 
-class _RowView(object):
+class _RowView(Mapping):
     """
     Lightweight read-only view over a row tuple, supporting dict-like access.
     Shares a single index map across all rows from the same result set,
     avoiding per-row dict allocation overhead.
+
+    Implements the :class:`collections.abc.Mapping` protocol, providing
+    ``__getitem__``, ``__iter__``, ``__len__``, ``get``, ``keys``,
+    ``values``, ``items``, and ``__contains__`` for free.
     """
 
     __slots__ = ("_row", "_index_map")
@@ -68,6 +72,12 @@ def __init__(self, row, index_map):
     def __getitem__(self, key):
         return self._row[self._index_map[key]]
 
+    def __iter__(self):
+        return iter(self._index_map)
+
+    def __len__(self):
+        return len(self._index_map)
+
     def get(self, key, default=None):
         idx = self._index_map.get(key)
         if idx is not None:
@@ -1132,8 +1142,6 @@ class KeyspaceMetadata(object):
         "virtual",
         "graph_engine",
         "_exc_info",
-        "__dict__",
-        "__weakref__",
     )
 
     def __init__(
@@ -1306,8 +1314,6 @@ class UserType(object):
         "name",
         "field_names",
         "field_types",
-        "__dict__",
-        "__weakref__",
     )
 
     def __init__(self, keyspace, name, field_names, field_types):
@@ -1389,8 +1395,6 @@ class Aggregate(object):
         "initial_condition",
         "return_type",
         "deterministic",
-        "__dict__",
-        "__weakref__",
     )
 
     def __init__(
@@ -1509,8 +1513,6 @@ class Function(object):
         "deterministic",
         "monotonic",
         "monotonic_on",
-        "__dict__",
-        "__weakref__",
     )
 
     def __init__(
@@ -1647,8 +1649,6 @@ class TableMetadata(object):
         "_exc_info",
         "virtual",
         "extensions",
-        "__dict__",
-        "__weakref__",
     )
 
     compaction_options = {
@@ -2098,8 +2098,6 @@ class ColumnMetadata(object):
         "is_static",
         "is_reversed",
         "_cass_type",
-        "__dict__",
-        "__weakref__",
     )
 
     def __init__(
@@ -2140,8 +2138,6 @@ class IndexMetadata(object):
         "name",
         "kind",
         "index_options",
-        "__dict__",
-        "__weakref__",
     )
 
     def __init__(self, keyspace_name, table_name, index_name, kind, index_options):
@@ -2391,7 +2387,7 @@ class TriggerMetadata(object):
         table.
     """
 
-    __slots__ = ("table", "name", "options", "__dict__", "__weakref__")
+    __slots__ = ("table", "name", "options")
 
     def __init__(self, table_metadata, trigger_name, options=None):
         self.table = table_metadata
@@ -3193,6 +3189,9 @@ class SchemaParserV3(SchemaParserV22):
 
     _SELECT_KEYSPACES = "SELECT * FROM system_schema.keyspaces"
     _SELECT_TABLES = "SELECT * FROM system_schema.tables"
+    # Only fetch the columns used by _build_column_metadata / _build_table_columns.
+    # Keep this list in sync with those methods; subclasses that need extra
+    # columns should override _SELECT_COLUMNS.
     _SELECT_COLUMNS = "SELECT keyspace_name, table_name, column_name, clustering_order, kind, position, type FROM system_schema.columns"
     _SELECT_INDEXES = "SELECT * FROM system_schema.indexes"
     _SELECT_TRIGGERS = "SELECT * FROM system_schema.triggers"
@@ -4324,8 +4323,6 @@ class MaterializedViewMetadata(object):
         "where_clause",
         "options",
         "extensions",
-        "__dict__",
-        "__weakref__",
     )
 
     def __init__(