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============
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())
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