Merge lp:~jkakar/storm/is-empty-strips-order-by into lp:storm

[Jamu Kakar]

This branch introduces the following changes:

- ResultSet.is_empty strips order by clauses, since they're
  unnecessary for this operation.

[Robert Collins (lifeless)]

Looks good except for the typo ;)

review: Approve

[lp:~jkakar/storm/is-empty-strips-order-by]

362. By Jamu Kakar on 2010-07-22

- Fix a typo in a docstring.
- Unpack the list of captured statements to ensure there is only
  one.

[Kevin McDermott (bigkevmcd)]

As discussed "ORDER BY" could not appear in a whole load of SQL statements, can we check that there's only one query executed, and that "ORDER BY" doesn't appear.

Oh, and as Robert points out "natching" ;-)

review: Approve

=== modified file 'storm/store.py'
--- storm/store.py	2010-02-10 11:29:39 +0000
+++ storm/store.py	2010-07-22 16:18:43 +0000
@@ -1055,6 +1055,7 @@
         """Return true if this L{ResultSet} contains no results."""
         subselect = self._get_select()
         subselect.limit = 1
+        subselect.order_by = Undef
         select = Select(1, tables=Alias(subselect, "_tmp"), limit=1)
         result = self._store._connection.execute(select)
         return (not result.get_one())

=== modified file 'tests/store/base.py'
--- tests/store/base.py	2010-06-01 08:31:53 +0000
+++ tests/store/base.py	2010-07-22 16:18:43 +0000
@@ -30,7 +30,8 @@
 from storm.properties import PropertyPublisherMeta, Decimal
 from storm.variables import PickleVariable
 from storm.expr import (
-    Asc, Desc, Select, LeftJoin, SQL, Count, Sum, Avg, And, Or, Eq, Lower)
+    Asc, Desc, Select, LeftJoin, SQL, Count, Sum, Avg, And, Or, Eq, Lower,
+    compile)
 from storm.variables import Variable, UnicodeVariable, IntVariable
 from storm.info import get_obj_info, ClassAlias
 from storm.exceptions import (
@@ -513,6 +514,26 @@
         result = self.store.find(Foo, id=30)
         self.assertEquals(result.is_empty(), False)
 
+    def test_wb_is_empty_strips_order_by(self):
+        """
+        L{ResultSet.is_empty} strips the C{ORDER BY} clause, if one is
+        present, since it isn't required to actually determine if a result set
+        has any matching rows.  This should provide some performance
+        improvement when the ordered result set would be large.
+        """
+        statements = []
+        original_execute = self.store._connection.execute
+        def execute(expr):
+            statements.append(compile(expr))
+            return original_execute(expr)
+        self.store._connection.execute = execute
+
+        result = self.store.find(Foo, Foo.id == 300)
+        result.order_by(Foo.id)
+        self.assertEqual(True, result.is_empty())
+        [statement] = statements
+        self.assertNotIn("ORDER BY", statement)
+
     def test_is_empty_with_composed_key(self):
         result = self.store.find(Link, foo_id=300, bar_id=3000)
         self.assertEquals(result.is_empty(), True)