redshift: also try to get schema from svv_columns

data_diff/databases/redshift.py

@@ -122,6 +122,38 @@ def query_pg_get_cols(self, path: DbPath) -> Dict[str, tuple]:
 
         return schema_dict
 
+    def select_svv_columns_schema(self, path: DbPath) -> Dict[str, tuple]:
+        database, schema, table = self._normalize_table_path(path)
+
+        db_clause = ""
+        if database:
+            db_clause = f" AND table_catalog = '{database.lower()}'"
+
+        return (
+            f"""
+            select
+            	distinct
+                column_name,
+            	data_type,
+            	datetime_precision,
+            	numeric_precision,
+            	numeric_scale
+            from
+            	svv_columns
+            where table_name = '{table.lower()}' and table_schema = '{schema.lower()}'
+            """
+            + db_clause
+        )
+
+    def query_svv_columns(self, path: DbPath) -> Dict[str, tuple]:
+        rows = self.query(self.select_svv_columns_schema(path), list)
+        if not rows:
+            raise RuntimeError(f"{self.name}: Table '{'.'.join(path)}' does not exist, or has no columns")
+
+        d = {r[0]: r for r in rows}
+        assert len(d) == len(rows)
+        return d
+
     # when using a non-information_schema source, strip (N) from type(N) etc. to match
     # typical information_schema output
     def _normalize_schema_info(self, rows) -> Dict[str, tuple]:
@@ -150,7 +182,10 @@ def query_table_schema(self, path: DbPath) -> Dict[str, tuple]:
             try:
                 return self.query_external_table_schema(path)
             except RuntimeError:
-                return self.query_pg_get_cols(path)
+                try:
+                    return self.query_pg_get_cols(path)
+                except Exception:
+                    return self.query_svv_columns(path)
 
     def _normalize_table_path(self, path: DbPath) -> DbPath:
         if len(path) == 1: