query: 0.19.0-rc.0 regression - Query with resolution. Graphs are dotted instead of lines

[ahurtaud]

Thanos, Prometheus and Golang version used:
thanos v0.19.0-rc.0
prometheus v2.25.0
grafana v7.4.3

What happened:
We are validating 0.19.0-rc.0, by just upgrading the version, most of our grafana dashboards are becoming dotted graphs instead of lines. I suspect some resolution/step issue. As on thanos UI, it happens only when we set a resolution to 30s (our scrape interval)

Step is a default behaviour we have in our grafana dahsboards.

What you expected to happen:
Like v0.18.0 and like prometheus UI, having connected lines for the query.

How to reproduce it (as minimally and precisely as possible):
Scrape something at 30s interval. Simple query raw metrics with a resolution of 30s. Compare Thanos replies vs prometheus reply.

[bwplotka]

Interesting. Thanks for reporting.

I wonder if it has something to do with the step changes we did with @hitanshu-mehta (#3740) 🤗

[hitanshu-mehta]

Yes. Resolution set in UI is overwritten every time by this line.

thanos/pkg/api/query/v1.go

Line 243 in dfcfbe4

d := time.Duration(math.Max(float64(rangeSeconds/250), float64(defaultRangeQueryStep/time.Second))) * time.Second

I can fix this :)