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