--- description: "Technical details on how the optimizer introspects queries and builds optimizations." --- # Technical details The optimizer uses the `GraphQLResolveInfo` and GraphQL AST to introspect the desired query, and construct [queryset.only()][only], [queryset.select_related()][select], and [queryset.prefetch_related()][prefetch] statements for the resolver queryset. The queryset is then "marked as optimized" in the [queryset hints](1) by setting a key defined by the `OPTIMIZER_MARK` setting. > Queryset hints are designed to be used in multi-database routing, so this > is a slightly hacky way to ensuring the mark is retained when the > queryset is cloned. It is relatively safe since multi-database routers > should accept the hints as **kwargs, and can ignore this extra hint. [only]: https://docs.djangoproject.com/en/dev/ref/models/querysets/#only [select]: https://docs.djangoproject.com/en/dev/ref/models/querysets/#select-related [prefetch]: https://docs.djangoproject.com/en/dev/ref/models/querysets/#prefetch-related [extensions]: https://github.com/graphql-python/graphql-core/blob/0c93b8452eed38d4f800c7e71cf6f3f3758cd1c6/src/graphql/type/schema.py#L123 [WeakKeyDictionary]: https://docs.python.org/3/library/weakref.html#weakref.WeakKeyDictionary [Inline fragments]: https://graphql.org/learn/queries/#inline-fragments [queryset hints]: https://docs.djangoproject.com/en/4.2/topics/db/multi-db/#hints