MyRocks and Bloom Filters

Bloom filters are used to reduce read amplification. Bloom filter can be set on a per-column family basis ( see myrocks-column-families).

Bloom filter parameters

• How many bits to use
• whole_key_filtering=true/false (TODO what is that?)
• Whether the bloom filter is for the entire key or for the prefix. In case of a prefix, you need to look at the index definition and compute the desired prefix length.

Computing prefix length

• It's 4 bytes for index_nr
• Then it is key_length as shown by EXPLAIN.

Enabling bloom filter:

To enable 10-bit bloom filter for 4-byte prefix length for column family "cf1", put this into my.cnf:

rocksdb_override_cf_options='cf1={block_based_table_factory={filter_policy=bloomfilter:10:false;whole_key_filtering=0;};prefix_extractor=capped:4};'

and restart the server.

Check if the column family actually uses the bloom filter:

<<code:sql>>
select * 
from information_schema.rocksdb_cf_options 
where 
  cf_name='cf1' and
  option_type IN ('TABLE_FACTORY::FILTER_POLICY','PREFIX_EXTRACTOR');
<</code>>

<code>> +---------+------------------------------+----------------------------+

+---------+------------------------------+----------------------------+

+---------+------------------------------+----------------------------+ <</code>>