An Index enables efficient store queries using filters that the index covers. An index may be in one of 3 states:

* Not built: the index neither exists in memory nor is it
  persisted. It can't be used.

* Persisted but not ready: the index has been built and persisted
  (e.g., to disk) but has not been loaded into memory and therefore
  can't be used.

* Ready: the index is loaded into memory (either because it was
  just built in memory, or because it was read from its persisted
  form) and can be used.

Index is referenced in 1 repository