Filter defines an 'effective filter' to use. An 'effective filter' if defined will be used to generate per-table filter block. The filter name will be stored on disk. During reads LevelDB will try to find matching filter from 'effective filter' and 'alternative filters'.

Filter can be changed after a DB has been created. It is recommended to put old filter to the 'alternative filters' to mitigate lack of filter during transition period.

A filter is used to reduce disk reads when looking for a specific key.

The default value is nil.