Add a doc about the DEFAULT_QUERY_DELTA setting of datagrepper

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

docs/datanommer_datagrepper/default_delta.rst

@@ -0,0 +1,18 @@
+Default delta
+=============
+
+Checking the current status of datagrepper, we realized that not specifying a
+`delta` value in the URL led to timeouts while specifying one, makes datagrepper
+return properly.
+
+Investigating the configuration options of datagrepper, we found out that
+there is a `DEFAULT_QUERY_DELTA` configuration key that allows to specify a
+default delta value when one is not specified.
+
+Just setting that configuration key to ``60*60*24*3`` (ie: 3 days) improves the
+datagrepper performances quite a bit (as in queries actually return instead of
+timing out).
+
+
+That configuration change, does break the API a little bit as with it, it will
+limit the messages returned to the last 3 days.

docs/datanommer_datagrepper/index.rst

@@ -28,6 +28,7 @@ Here is the list of ideas/things we looked at:
 .. toctree::
     :maxdepth: 1
 
+    default_delta
     pg_stat_statements
     pg_partitioning
     pg_timescaledb