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_r191262164
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File path: src/ddocs/ddocs.rst
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@@ -166,6 +184,82 @@ JavaScript:
}
}
+The ``_stats`` function will also work with "pre-aggregated" values from a map
+phase. A map function that emits an object containing ``sum``, ``min``,
``max``,
+``count``, and ``sumsqr`` keys and numeric values for each can use the
+``_stats`` function to combine these results with the data from other
documents.
+The emitted object may contain other keys (these are ignored by the reducer),
+and it is also possible to mix raw numeric values and pre-aggregated objects
+in a single view and obtain the correct aggregated statistics.
+
+Finally, ``_stats`` can operate on key-value pairs where each value is an array
+comprised of numbers or pre-aggregated objects. In this case **every** value
+emitted from the map function must be an array, and the arrays must all be the
+same length, as ``_stats`` will compute the statistical quantities above
+*independently* for each element in the array. Users who want to compute
+statistics on multiple values from a single document should either ``emit``
each
+value into the index separately, or compute the statistics for the set of
values
+using the JavaScript example above and emit a pre-aggregated object.
+
+.. data:: _sum
+
+In it simplest variation, ``_sum`` sums the numeric values associated with each
+key, as in the following JavaScript:
+
+.. code-block:: javascript
+
+ // could be replaced by _sum
+ function(keys, values) {
+ return sum(values);
+ }
+
+As with ``_stats``, the ``_sum`` function offers a number of extended
+capabilities. The _sum function requires that map values be numbers, arrays of
Review comment:
Missing double quotes around `_sum` on this line.
capabilities. The ``_sum`` function ...
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