diff --git a/core/lib/Drupal/Core/Entity/Query/Sql/pgsql/QueryFactory.php b/core/lib/Drupal/Core/Entity/Query/Sql/pgsql/QueryFactory.php
index cf7fdb5..fb43f13 100644
--- a/core/lib/Drupal/Core/Entity/Query/Sql/pgsql/QueryFactory.php
+++ b/core/lib/Drupal/Core/Entity/Query/Sql/pgsql/QueryFactory.php
@@ -10,6 +10,21 @@
 
 use Drupal\Core\Entity\Query\Sql\QueryFactory as BaseQueryFactory;
 
+/**
+ * PostgreSQL specific entity query implementation.
+ *
+ * To add a new query implementation extending the default SQL one, add
+ * a service definition like pgsql.entity.query.sql and a factory class like
+ * this. The system will automatically find the relevant Query, QueryAggregate,
+ * Condition, ConditionAggregate, Tables classes in this namespace, in the
+ * namespace of the parent class and so on. So after creating an empty query
+ * factory class like this, it is possible to just drop in a class extending
+ * the base class in this namespace and it will be used automatically but it
+ * is optional: if a class is not extended the relevant default is used.
+ *
+ * @see \Drupal\Core\Entity\Query\QueryBase::getNamespaces()
+ * @see \Drupal\Core\Entity\Query\QueryBase::getClass()
+ */
 class QueryFactory extends BaseQueryFactory {
 
 }