diff -u b/core/lib/Drupal/Core/Database/Driver/pgsql/Select.php b/core/lib/Drupal/Core/Database/Driver/pgsql/Select.php
--- b/core/lib/Drupal/Core/Database/Driver/pgsql/Select.php
+++ b/core/lib/Drupal/Core/Database/Driver/pgsql/Select.php
@@ -53,6 +53,9 @@
    */
   public function orderBy($field, $direction = 'ASC') {
     // Only allow ASC and DESC, default to ASC.
+    // Emulate MySQL default behavior to sort NULL values first for ascending,
+    // and last for descending.
+    // @see http://www.postgresql.org/docs/9.3/static/queries-order.html
     $direction = strtoupper($direction) == 'DESC' ? 'DESC NULLS LAST' : 'ASC NULLS FIRST';
     $this->order[$field] = $direction;
 
diff -u b/core/modules/system/src/Tests/Database/SelectComplexTest.php b/core/modules/system/src/Tests/Database/SelectComplexTest.php
--- b/core/modules/system/src/Tests/Database/SelectComplexTest.php
+++ b/core/modules/system/src/Tests/Database/SelectComplexTest.php
@@ -221,7 +221,8 @@
 
     // Check that the ordering clause is handled properly.
     $orderby = $query->getOrderBy();
-    // The orderby string is different for postgreSQL.
+    // The orderby string is different for PostgreSQL.
+    // @see Drupal\Core\Database\Driver\pgsql\Select::orderBy()
     $db_type = Database::getConnection()->databaseType();
     $this->assertEqual($orderby['name'], ($db_type == 'pgsql' ? 'ASC NULLS FIRST' : 'ASC'), 'Query correctly sets ordering clause.');
     $orderby = $count->getOrderBy();