Expected different result from count()

[codesome]

Bug Report

What did you do?
Spin up prometheus (with config at the end). All the targets are down.

up{instance="localhost:9100",job="node_exporter",test="tst"} 0
up{instance="localhost:9998",job="dummy2",test="tst"} 0
up{instance="localhost:9999",job="dummy",test="tst"} 0

Ran the query count(up{test=~".*tst.*"} == 1) by (test)

What did you expect to see?
A series/sample with count (value) as 0.

What did you see instead? Under which circumstances?
No data as output. This happens only when all the up series are 0.
This comes as nil in the code, hence any expression including that part becomes nil and interferes with alerting too. For example an alerting rule with count(up{test=~".*tst.*"} == 1) by (test) == 0 would never fire.

• Prometheus version:

  master @ 3e6b9d4 (post 2.4.3)

• Prometheus configuration file:

global:
  scrape_interval:     5s
  evaluation_interval: 30s

scrape_configs:
- job_name: node_exporter
  static_configs:
  - targets: ['localhost:9100']
    labels:
      test: tst
- job_name: dummy
  static_configs:
  - targets: ['localhost:9999']
    labels:
      test: tst
- job_name: dummy2
  static_configs:
  - targets: ['localhost:9998']
    labels:
      test: tst

[brian-brazil]

See https://www.robustperception.io/why-can-countx-5-not-return-0

[codesome]

/cc @metalmatze

[codesome]

Thanks @brian-brazil for the blog post!

[metalmatze]

Thanks for further looking into at @codesome. 💯