sql: Move approx_len to backend

Orange/data/sql/backend/base.py

@@ -130,6 +130,19 @@ def create_variable(self, field_name, field_metadata,
         """
         raise NotImplementedError
 
+    def count_approx(self, query):
+        """Return estimated number of rows returned by query.
+
+        Parameters
+        ----------
+        query : str
+
+        Returns
+        -------
+        Approximate number of rows
+        """
+        raise NotImplementedError
+
     # query related methods
 
     def create_sql_query(

Orange/data/sql/backend/postgres.py

@@ -1,4 +1,5 @@
 import logging
+import re
 import warnings
 from contextlib import contextmanager
 from time import time
@@ -7,7 +8,7 @@
 from psycopg2.pool import ThreadedConnectionPool
 
 from Orange.data import ContinuousVariable, DiscreteVariable, StringVariable, TimeVariable
-from Orange.data.sql.backend.base import Backend, TableDesc, ToSql, BackendError
+from Orange.data.sql.backend.base import Backend, ToSql, BackendError
 
 log = logging.getLogger(__name__)
 
@@ -167,6 +168,12 @@ def _guess_variable(self, field_name, field_metadata, inspect_table):
 
         return StringVariable(field_name)
 
+    def count_approx(self, query):
+        sql = "EXPLAIN " + query
+        with self.execute_sql_query(sql) as cur:
+            s = ''.join(row[0] for row in cur.fetchall())
+        return int(re.findall(r'rows=(\d*)', s)[0])
+
     def __getstate__(self):
         # Drop connection_pool from state as it cannot be pickled
         state = dict(self.__dict__)

Orange/data/sql/table.py

@@ -264,13 +264,20 @@ def _count_rows(self):
     def approx_len(self, get_exact=False):
         if self._cached__len__ is not None:
             return self._cached__len__
-        sql = "EXPLAIN " + self._sql_query(["*"])
-        with self.backend.execute_sql_query(sql) as cur:
-            s = ''.join(row[0] for row in cur.fetchall())
-        alen = int(re.findall('rows=(\d*)', s)[0])
-        if get_exact:
-            threading.Thread(target=len, args=(self,)).start()
-        return alen
+
+        approx_len = None
+        try:
+            query = self._sql_query(["*"])
+            approx_len = self.backend.count_approx(query)
+            if get_exact:
+                threading.Thread(target=len, args=(self,)).start()
+        except NotImplementedError:
+            pass
+
+        if approx_len is None:
+            approx_len = len(self)
+
+        return approx_len
 
     _X = None
     _Y = None