Missing a data point as adjusting query step

[yhkim713]

Bug Report

What did you do?
Adjusting step param(required from query_range API) to make my graph more pretty but some points are missing unexpectedly.
A data scrape interval is 10sec

a first query is

increase(exception[1m] with a step 59s
--- it's json output is 
values:Array[7]
0:Array[1544668439,0]
1:Array[1544668498,0]
2:Array[1544668557,1.2]
3:Array[1544668616,0]
4:Array[1544668675,0]
5:Array[1544668734,0]
6:Array[1544668793,0]

and second query is

increase(exception[1m]) with a step 60s
--- it's json output is
values:Array[6]
0:Array[1544668440,0]
1:Array[1544668500,0]
2:Array[1544668560,0] // here, I expected that it should have a value
3:Array[1544668620,0]
4:Array[1544668680,0]
5:Array[1544668740,0]

Environment

• Prometheus version:
  prometheus, version 2.1.0 (branch: HEAD, revision: 85f23d8)
  build user: root@6e784304d3ff
  build date: 20180119-12:01:23
  go version: go1.9.2

[simonpasquier]

As stated in the documentation, Prometheus doesn't aim for 100% accuracy. This is why rate()/increase(), especially applied to slowly increasing counters, don't always yield the same results depending on the step interval. See #3746 for a (much) longer discussion and the current take from the Prometheus team.

I'm closing it for now. If you have further questions, please use our user mailing list, which you can also search.

[yhkim713]

thank you for your detailed answer and the link

[brian-brazil]

The issue here is more that your step and range are about the same, you want a range that's larger than your step to avoid things like this.