(improvement) metadata: replace dict_factory with lightweight _RowView

Replace dict_factory in _SchemaParser._handle_results and
get_column_from_system_local with _row_factory, eliminating per-row
dict allocation during schema parsing.

Also refactor SchemaParserV4._build_keyspace_metadata_internal to read
from the row without mutating it, since _RowView is read-only.

Note: V22-only dict_factory call sites are left unchanged as they do not
affect the V3/V4 code path (V3 and V4 fully override _query_all).

Author: mykaul

cassandra/metadata.py

@@ -2068,7 +2068,7 @@ def get_next_pages():
                         yield next_result.parsed_rows
 
                 result.parsed_rows += itertools.chain(*get_next_pages())
-            return dict_factory(result.column_names, result.parsed_rows) if result else []
+            return _row_factory(result.column_names, result.parsed_rows) if result else []
         else:
             raise result
 
@@ -3120,11 +3120,12 @@ def get_all_keyspaces(self):
 
     @staticmethod
     def _build_keyspace_metadata_internal(row):
-        # necessary fields that aren't int virtual ks
-        row["durable_writes"] = row.get("durable_writes", None)
-        row["replication"] = row.get("replication", {})
-        row["replication"]["class"] = row["replication"].get("class", None)
-        return super(SchemaParserV4, SchemaParserV4)._build_keyspace_metadata_internal(row)
+        # Read without mutating the row, since _RowView is read-only
+        name = row["keyspace_name"]
+        durable_writes = row.get("durable_writes", None)
+        replication = dict(row.get("replication")) if row.get("replication") else {}
+        replication_class = replication.pop("class") if 'class' in replication else None
+        return KeyspaceMetadata(name, durable_writes, replication_class, replication)
 
 
 class SchemaParserDSE67(SchemaParserV4):
@@ -3190,7 +3191,7 @@ def get_table(self, keyspaces, keyspace, table):
     def _build_keyspace_metadata_internal(row):
         name = row["keyspace_name"]
         durable_writes = row.get("durable_writes", None)
-        replication = dict(row.get("replication")) if 'replication' in row else {}
+        replication = dict(row.get("replication")) if row.get("replication") else {}
         replication_class = replication.pop("class") if 'class' in replication else None
         graph_engine = row.get("graph_engine", None)
         return KeyspaceMetadata(name, durable_writes, replication_class, replication, graph_engine)
@@ -3528,7 +3529,7 @@ def get_column_from_system_local(connection, column_name: str, timeout, metadata
         , timeout=timeout, fail_on_error=False)
     if not success or not local_result.parsed_rows:
         return ""
-    local_rows = dict_factory(local_result.column_names, local_result.parsed_rows)
+    local_rows = _row_factory(local_result.column_names, local_result.parsed_rows)
     local_row = local_rows[0]
     return local_row.get(column_name)