flimzy commented on a change in pull request #279: Expand docs on builtin
reducers, add _approx_count_distinct
URL:
https://github.com/apache/couchdb-documentation/pull/279#discussion_r191261676
##########
File path: src/ddocs/ddocs.rst
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@@ -116,17 +116,25 @@ single value - which could be an array or similar object.
Built-in Reduce Functions
^^^^^^^^^^^^^^^^^^^^^^^^^
-Additionally, CouchDB has three built-in reduce functions. These are
implemented
-in Erlang and run inside CouchDB, so they are much faster than the equivalent
-JavaScript functions: ``_sum``, ``_count`` and ``_stats``. Their equivalents in
-JavaScript:
+Additionally, CouchDB has a set of built-in reduce functions. These are
+implemented in Erlang and run inside CouchDB, so they are much faster than the
+equivalent JavaScript functions.
-.. code-block:: javascript
+.. data:: _approx_count_distinct
- // could be replaced by _sum
- function(keys, values) {
- return sum(values);
- }
+Since version `2.2.0`.
+
+Aproximates the number of distinct keys in a view index using a variant of the
+HyperLogLog algorithm. This algorithm enables an efficient, parallelizable
Review comment:
Perhaps we can provide a link to an explanation of this algorithm for those
(like me) who have never heard of it? Maybe just
https://en.wikipedia.org/wiki/HyperLogLog ?
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