Use per-table DTE to get the table comments

Signed-off-by: Christophe Bornet <cbornet@hotmail.com>

src/databricks/sqlalchemy/base.py

@@ -1,9 +1,8 @@
-from typing import Any, List, Optional, Dict, Union, Iterable, Tuple
+from typing import Any, List, Optional, Dict, Union
 
 import databricks.sqlalchemy._ddl as dialect_ddl_impl
 import databricks.sqlalchemy._types as dialect_type_impl
 from databricks import sql
-from databricks.sqlalchemy._information_schema import tables
 from databricks.sqlalchemy._parse import (
     _describe_table_extended_result_to_dict_list,
     _match_table_not_found_string,
@@ -15,16 +14,15 @@
 )
 
 import sqlalchemy
-from sqlalchemy import DDL, event, select, bindparam, exc
+from sqlalchemy import DDL, event, exc
 from sqlalchemy.engine import Connection, Engine, default, reflection
 from sqlalchemy.engine.interfaces import (
     ReflectedForeignKeyConstraint,
     ReflectedPrimaryKeyConstraint,
     ReflectedColumn,
     ReflectedTableComment,
-    TableKey,
 )
-from sqlalchemy.engine.reflection import ReflectionDefaults, ObjectKind, ObjectScope
+from sqlalchemy.engine.reflection import ReflectionDefaults
 from sqlalchemy.exc import DatabaseError, SQLAlchemyError
 
 try:
@@ -376,86 +374,29 @@ def get_schema_names(self, connection, **kw):
         schema_list = [row[0] for row in result]
         return schema_list
 
-    def get_multi_table_comment(
-        self,
-        connection,
-        schema=None,
-        filter_names=None,
-        scope=ObjectScope.ANY,
-        kind=ObjectKind.ANY,
-        **kw,
-    ) -> Iterable[Tuple[TableKey, ReflectedTableComment]]:
-        result = []
-        _schema = schema or self.schema
-        if ObjectScope.DEFAULT in scope:
-            query = (
-                select(tables.c.table_name, tables.c.comment)
-                .select_from(tables)
-                .where(
-                    tables.c.table_catalog == self.catalog,
-                    tables.c.table_schema == _schema,
-                )
-            )
-
-            if ObjectKind.ANY not in kind:
-                where_in = set()
-                if ObjectKind.TABLE in kind:
-                    where_in.update(
-                        ["BASE TABLE", "MANAGED", "EXTERNAL", "STREAMING_TABLE"]
-                    )
-                if ObjectKind.VIEW in kind:
-                    where_in.update(["VIEW"])
-                if ObjectKind.MATERIALIZED_VIEW in kind:
-                    where_in.update(["MATERIALIZED_VIEW"])
-                query = query.where(tables.c.table_type.in_(where_in))
-
-            if filter_names:
-                query = query.where(tables.c.table_name.in_(bindparam("filter_names")))
-                result = connection.execute(
-                    query, {"filter_names": [f.lower() for f in filter_names]}
-                )
-            else:
-                result = connection.execute(query)
-
-        if ObjectScope.TEMPORARY in scope and ObjectKind.VIEW in kind:
-            result = list(result)
-            temp_views = self.get_view_names(connection, schema, only_temp=True)
-            if filter_names:
-                temp_views = set(temp_views).intersection(
-                    [f.lower() for f in filter_names]
-                )
-            result.extend(zip(temp_views, [None] * len(temp_views)))
-
-        # TODO: figure out how to return sqlalchemy.interfaces in a way that mypy respects
-        return (
-            (
-                (schema, table),
-                {"text": comment}
-                if comment is not None
-                else ReflectionDefaults.table_comment(),
-            )
-            for table, comment in result
-        )  # type: ignore
-
+    @reflection.cache
     def get_table_comment(
         self,
         connection: Connection,
         table_name: str,
         schema: Optional[str] = None,
         **kw: Any,
     ) -> ReflectedTableComment:
-        data = self.get_multi_table_comment(
-            connection,
-            schema,
-            [table_name],
-            **kw,
+        result = self._describe_table_extended(
+            connection=connection,
+            table_name=table_name,
+            schema_name=schema,
+        )
+        # Type ignore is because mypy knows that self._describe_table_extended *can*
+        # return None (even though it never will since expect_result defaults to True)
+        comment_row: Dict[str, str] = next(
+            filter(lambda r: r["col_name"] == "Comment", result), None
+        )  # type: ignore
+        return (
+            {"text": comment_row["data_type"]}
+            if comment_row
+            else ReflectionDefaults.table_comment()
         )
-        try:
-            return dict(data)[(schema, table_name.lower())]
-        except KeyError:
-            raise exc.NoSuchTableError(
-                f"{schema}.{table_name}" if schema else table_name
-            ) from None
 
 
 @event.listens_for(Engine, "do_connect")