This is an automated email from the ASF dual-hosted git repository.
fjy pushed a commit to branch 0.14.0-incubating
in repository https://gitbox.apache.org/repos/asf/incubator-druid.git
The following commit(s) were added to refs/heads/0.14.0-incubating by this push:
new 62a655a Add more approximate algorithm docs (#7195) (#7196)
62a655a is described below
commit 62a655a14ef5f5c409b6a1480f938c81de23d7e1
Author: Jonathan Wei <[email protected]>
AuthorDate: Tue Mar 5 17:06:38 2019 -0800
Add more approximate algorithm docs (#7195) (#7196)
---
.../extensions-core/approximate-histograms.md | 6 ++++-
docs/content/development/extensions.md | 2 +-
docs/content/querying/aggregations.md | 31 ++++++++++++++++++++--
3 files changed, 35 insertions(+), 4 deletions(-)
diff --git a/docs/content/development/extensions-core/approximate-histograms.md
b/docs/content/development/extensions-core/approximate-histograms.md
index ea9e17b..b60ff13 100644
--- a/docs/content/development/extensions-core/approximate-histograms.md
+++ b/docs/content/development/extensions-core/approximate-histograms.md
@@ -28,7 +28,11 @@ Make sure to
[include](../../operations/including-extensions.html) `druid-histog
The `druid-histogram` extension provides an approximate histogram aggregator
and a fixed buckets histogram aggregator.
-## Approximate Histogram aggregator
+## Approximate Histogram aggregator (Deprecated)
+
+<div class="note caution">
+The Approximate Histogram aggregator is deprecated. Please use <a
href="../extensions-core/datasketches-quantiles.html">DataSketches
Quantiles</a> instead which provides a superior distribution-independent
algorithm with formal error guarantees.
+</div>
This aggregator is based on
[http://jmlr.org/papers/volume11/ben-haim10a/ben-haim10a.pdf](http://jmlr.org/papers/volume11/ben-haim10a/ben-haim10a.pdf)
diff --git a/docs/content/development/extensions.md
b/docs/content/development/extensions.md
index d5f5531..91f6add 100644
--- a/docs/content/development/extensions.md
+++ b/docs/content/development/extensions.md
@@ -47,7 +47,7 @@ Core extensions are maintained by Druid committers.
|druid-caffeine-cache|A local cache implementation backed by
Caffeine.|[link](../development/extensions-core/caffeine-cache.html)|
|druid-datasketches|Support for approximate counts and set operations with
[DataSketches](http://datasketches.github.io/).|[link](../development/extensions-core/datasketches-extension.html)|
|druid-hdfs-storage|HDFS deep
storage.|[link](../development/extensions-core/hdfs.html)|
-|druid-histogram|Approximate histograms and quantiles
aggregator.|[link](../development/extensions-core/approximate-histograms.html)|
+|druid-histogram|Approximate histograms and quantiles aggregator. Deprecated,
please use the [DataSketches quantiles
aggregator](../development/extensions-core/datasketches-quantiles.html) from
the `druid-datasketches` extension
instead.|[link](../development/extensions-core/approximate-histograms.html)|
|druid-kafka-eight|Kafka ingest firehose (high level consumer) for realtime
nodes.|[link](../development/extensions-core/kafka-eight-firehose.html)|
|druid-kafka-extraction-namespace|Kafka-based namespaced lookup. Requires
namespace lookup
extension.|[link](../development/extensions-core/kafka-extraction-namespace.html)|
|druid-kafka-indexing-service|Supervised exactly-once Kafka ingestion for the
indexing service.|[link](../development/extensions-core/kafka-ingestion.html)|
diff --git a/docs/content/querying/aggregations.md
b/docs/content/querying/aggregations.md
index c2d2c72..a9a819b 100644
--- a/docs/content/querying/aggregations.md
+++ b/docs/content/querying/aggregations.md
@@ -277,10 +277,16 @@ The [DataSketches Theta
Sketch](../development/extensions-core/datasketches-thet
The [DataSketches HLL
Sketch](../development/extensions-core/datasketches-hll.html)
extension-provided aggregator gives distinct count estimates using the
HyperLogLog algorithm. The HLL Sketch is faster and requires less storage than
the Theta Sketch, but does not support intersection or difference operations.
-#### Cardinality/HyperUnique
+#### Cardinality/HyperUnique (Deprecated)
+
+<div class="note caution">
+The Cardinality and HyperUnique aggregators are deprecated. Please use <a
href="../extensions-core/datasketches-hll.html">DataSketches HLL Sketch</a>
instead.
+</div>
The [Cardinality and HyperUnique](../hll-old.html) aggregators are older
aggregator implementations available by default in Druid that also provide
distinct count estimates using the HyperLogLog algorithm. The newer
[DataSketches HLL Sketch](../development/extensions-core/datasketches-hll.html)
extension-provided aggregator has superior accuracy and performance and is
recommended instead.
+The DataSketches team has published a [comparison
study](https://datasketches.github.io/docs/HLL/HllSketchVsDruidHyperLogLogCollector.html)
between Druid's original HLL algorithm and the DataSketches HLL algorithm.
Based on the demonstrated advantages of the DataSketches implementation, we
have deprecated Druid's original HLL aggregator.
+
Please note that DataSketches HLL aggregators and `hyperUnique` aggregators
are not mutually compatible.
### Histograms and quantiles
@@ -289,10 +295,31 @@ Please note that DataSketches HLL aggregators and
`hyperUnique` aggregators are
The [DataSketches Quantiles
Sketch](../development/extensions-core/datasketches-quantiles.html)
extension-provided aggregator provides quantile estimates and histogram
approximations using the numeric quantiles DoublesSketch from the
[datasketches](http://datasketches.github.io/) library.
-#### Approximate Histogram
+We recommend this aggregator in general for quantiles/histogram use cases, as
it provides formal error bounds and has distribution-independent accuracy.
+
+#### Fixed Buckets Histogram
+
+Druid also provides a [simple histogram
implementation]((../development/extensions-core/approxiate-histograms.html#fixed-buckets-histogram)
that uses a fixed range and fixed number of buckets with support for quantile
estimation, backed by an array of bucket count values.
+
+The fixed buckets histogram can perform well when the distribution of the
input data allows a small number of buckets to be used.
+
+We do not recommend the fixed buckets histogram for general use, as its
usefulness is extremely data dependent. However, it is made available for users
that have already identified use cases where a fixed buckets histogram is
suitable.
+
+#### Approximate Histogram (Deprecated)
The [Approximate
Histogram](../development/extensions-core/approxiate-histograms.html)
extension-provided aggregator also provides quantile estimates and histogram
approximations, based on
[http://jmlr.org/papers/volume11/ben-haim10a/ben-haim10a.pdf](http://jmlr.org/papers/volume11/ben-haim10a/ben-haim10a.pdf).
+The algorithm used by this deprecated aggregator is highly
distribution-dependent and its output is subject to serious distortions when
the input does not fit within the algorithm's limitations.
+
+A [study published by the DataSketches
team](https://datasketches.github.io/docs/Quantiles/DruidApproxHistogramStudy.html)
demonstrates some of the known failure modes of this algorithm:
+- The algorithm's quantile calculations can fail to provide results for a
large range of rank values (all ranks less than 0.89 in the example used in the
study), returning all zeroes instead.
+- The algorithm can completely fail to record spikes in the tail ends of the
distribution
+- In general, the histogram produced by the algorithm can deviate
significantly from the true histogram, with no bounds on the errors.
+
+It is not possible to determine a priori how well this aggregator will behave
for a given input stream, nor does the aggregator provide any indication that
serious distortions are present in the output.
+
+For these reasons, we have deprecated this aggregator and do not recommend its
use.
+
## Miscellaneous Aggregations
### Filtered Aggregator
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
