fix the schema-fetching problem for impala in sql_lab (apache#3906)

* fix the schema-fetching problem for impala in sql_lab

* delete redundant print

* remove blank lines...

* minior corrections

superset/db_engine_specs.py

@@ -148,6 +148,10 @@ def adjust_database_uri(cls, uri, selected_schema):
     def patch(cls):
         pass
 
+    @classmethod
+    def get_schema_names(cls, inspector):
+        return inspector.get_schema_names()
+
     @classmethod
     def get_table_names(cls, schema, inspector):
         return sorted(inspector.get_table_names(schema))
@@ -1092,6 +1096,12 @@ def convert_dttm(cls, target_type, dttm):
             return "{}'".format(dttm.strftime('%Y-%m-%d'))
         return "'{}'".format(dttm.strftime('%Y-%m-%d %H:%M:%S'))
 
+    @classmethod
+    def get_schema_names(cls, inspector):
+        schemas = [row[0] for row in inspector.engine.execute('SHOW SCHEMAS')
+                   if not row[0].startswith('_')]
+        return schemas
+
 
 engines = {
     o.engine: o for o in globals().values()

superset/models/core.py

@@ -726,7 +726,7 @@ def all_view_names(self, schema=None, force=False):
         return views
 
     def all_schema_names(self):
-        return sorted(self.inspector.get_schema_names())
+        return sorted(self.db_engine_spec.get_schema_names(self.inspector))
 
     @property
     def db_engine_spec(self):