clintropolis commented on a change in pull request #6397: Adds bloom filter
aggregator to 'druid-bloom-filters' extension
URL: https://github.com/apache/incubator-druid/pull/6397#discussion_r223499886
##########
File path: docs/content/development/extensions-core/bloom-filter.md
##########
@@ -42,4 +50,53 @@ Internally, this implementation of bloom filter uses
Murmur3 fast non-cryptograp
- 1 big endian int(That is how OutputStream works) for the number of longs in
the bitset
- big endian longs in the BloomKFilter bitset
-Note: `org.apache.hive.common.util.BloomKFilter` provides a serialize method
which can be used to serialize bloom filters to outputStream.
\ No newline at end of file
+Note: `org.apache.hive.common.util.BloomKFilter` provides a serialize method
which can be used to serialize bloom filters to outputStream.
+
+## Bloom Filter Query Aggregator
+Input for a `bloomKFilter` can also be created from a druid query with the
`bloom` aggregator.
+
+### JSON Specification of Bloom Filter Aggregator
+```json
+{
+ "type": "bloomFilter",
+ "name": <output_field_name>,
+ "maxNumEntries": <maximum_number_of_elements_for_BloomKFilter>
+ "field": <dimension_spec>
+ }
+```
+
+|Property |Description |required?
|
+|-------------------------|------------------------------|----------------------------------|
+|`type` |Aggregator Type. Should always be `bloom`|yes|
+|`name` |Output field name |yes|
+|`field` |[DimensionSpec](./../dimensionspecs.html) to add to
`org.apache.hive.common.util.BloomKFilter` | yes |
+|`maxNumEntries` |Maximum number of distinct values supported by
`org.apache.hive.common.util.BloomKFilter`, default `1500`| no |
Review comment:
Hmm, digging into it, in `BloomKFilter` the [false positive rate is not
controllable](https://github.com/apache/hive/blob/master/storage-api/src/java/org/apache/hive/common/util/BloomKFilter.java#L61)
in the manner of `BloomFilter`, and is fixed to the default of 5%. However I
guess that can be indirectly controlled by increasing the `maxNumEntries`,
though that's kind of lame. Having a higher cardinality than the value of
`maxNumEntries` will cause the false positive probability to reach 1,
constructing a useless bloom filter, so that should definitely be added to the
docs.
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