Fixed #10142 -- Added docs and an exploding error message to highlight an error present in the implementation of STDDEV_POP and VAR_POP in PostgreSQL 8.2-8.2.4 that will give incorrect answers (the database function mistakenly returns sample, rather than population deviation/variance). Thanks to Vinay Sajip <vinay_sajip@yahoo.co.uk> for the report, and Ian Kelly for pointing out the cause of the problem.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9804 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/backends/__init__.py

@@ -398,6 +398,15 @@ class BaseDatabaseOperations(object):
         # Default to a float
         return float(value)
 
+    def check_aggregate_support(self, aggregate_func):
+        """Check that the backend supports the provided aggregate
+
+        This is used on specific backends to rule out known aggregates
+        that are known to have faulty implementations. If the named
+        aggregate function has a known problem, the backend should
+        raise NotImplemented.
+        """
+        pass
 
 class BaseDatabaseIntrospection(object):
     """

django/db/backends/postgresql/operations.py

@@ -144,3 +144,14 @@ class DatabaseOperations(BaseDatabaseOperations):
 
     def prep_for_iexact_query(self, x):
         return x
+
+    def check_aggregate_support(self, aggregate):
+        """Check that the backend fully supports the provided aggregate.
+
+        The implementation of population statistics (STDDEV_POP and VAR_POP)
+        under Postgres 8.2 - 8.2.4 is known to be faulty. Raise
+        NotImplementedError if this is the database in use.
+        """
+        if aggregate.sql_function == 'STDDEV_POP' or aggregate.sql_function == 'VAR_POP':
+            if self.postgres_version[0] == 8 and self.postgres_version[1] == 2 and self.postgres_version[1] <= 4:
+                raise NotImplementedError('PostgreSQL 8.2 to 8.2.4 is known to have a faulty implementation of %s. Please upgrade your version of PostgreSQL.' % aggregate.sql_function)

django/db/models/aggregates.py

@@ -41,8 +41,12 @@ class Aggregate(object):
          * is_summary is a boolean that is set True if the aggregate is a
            summary value rather than an annotation.
         """
-        aggregate = getattr(query.aggregates_module, self.name)
+        klass = getattr(query.aggregates_module, self.name)
-        query.aggregate_select[alias] = aggregate(col, source=source, is_summary=is_summary, **self.extra)
+        aggregate = klass(col, source=source, is_summary=is_summary, **self.extra)
+        # Validate that the backend has a fully supported, correct
+        # implementation of this aggregate
+        query.connection.ops.check_aggregate_support(aggregate)
+        query.aggregate_select[alias] = aggregate
 
 class Avg(Aggregate):
     name = 'Avg'

docs/ref/databases.txt

@@ -13,6 +13,22 @@ This file describes some of the features that might be relevant to Django
 usage. Of course, it is not intended as a replacement for server-specific
 documentation or reference manuals.
 
+PostgreSQL notes
+================
+
+PostgreSQL 8.2 to 8.2.4
+-----------------------
+
+The implementation of the population statistics aggregates ``STDDEV_POP`` and
+``VAR_POP`` that shipped with PostgreSQL 8.2 to 8.2.4 are `known to be
+faulty`_. Users of these releases of PostgreSQL are advised to upgrade to
+`Release 8.2.5`_ or later. Django will raise a ``NotImplementedError`` if you
+attempt to use the ``StdDev(sample=False)`` or ``Variance(sample=False)``
+aggregate with an database backend falls within the affected release range.
+
+.. _known to be faulty: http://archives.postgresql.org/pgsql-bugs/2007-07/msg00046.php
+.. _Release 8.2.5: http://developer.postgresql.org/pgdocs/postgres/release-8-2-5.html
+
 .. _mysql-notes:
 
 MySQL notes