============
Field caches
============

The default (``filedb``) backend uses *field caches* in certain circumstances.
The field cache basically pre-computes the order of documents in the index to
speed up sorting and faceting.

Generating field caches can take time the first time you sort/facet on a large
index. The field cache is kept in memory (and by default written to disk when it
is generated) so subsequent sorted/faceted searches should be faster.

The default caching policy never expires field caches, so reused searchers and/or
sorting a lot of different fields could use up quite a bit of memory with large
indexes.


Customizing cache behaviour
===========================

(The following API examples refer to the default ``filedb`` backend.)

*By default*, Whoosh saves field caches to disk. To prevent a reader or searcher
from writing out field caches, do this before you start using it::

    searcher.set_caching_policy(save=False)

By default, if caches are written to disk they are saved in the index directory.
To tell a reader or searcher to save cache files to a different location, create
a storage object and pass it to the ``storage`` keyword argument::

    from whoosh.filedb.filestore import FileStorage

    mystorage = FileStorage("path/to/cachedir")
    reader.set_caching_policy(storage=mystorage)


Creating a custom caching policy
================================

Expert users who want to implement a custom caching policy (for example, to add
cache expiration) should subclass :class:`whoosh.filedb.fieldcache.FieldCachingPolicy`.
Then you can pass an instance of your policy object to the ``set_caching_policy``
method::

    searcher.set_caching_policy(MyPolicy())