Show offending labels in many-to-many error

[matthiasr]

Proposal

When showing the error message

Error executing query: many-to-many matching not allowed: matching labels must be unique on one side

include the labels, and potentially even the whole metric, that caused it.

Use case. Why is this important?

I just spent half an hour staring at a query trying to fix this issue; sometimes a very large set of time series has duplication in only a handful of them. Knowing more about the reason for the many-to-many error would have helped a lot.

This message is generated here:

prometheus/promql/engine.go

Line 1222 in 576ee4d

ev.errorf("many-to-many matching not allowed: matching labels must be unique on one side")

[trajano]

While we're waiting for a fix. how did you debug your query to find out?

[matthiasr]

I aggregated away different labels on both sides until I found which one is causing the issue, then thought about how many-to-many matches could arise based on how the metrics were generated and collected. As I wrote, there's no clever way but to think about it really hard.

[dnwe]

I hit the same issue. I just manually patched engine.go to print out details of the label collision and ran my queries through that to see which were the cause

[matthiasr]

Could you submit that patch?

…

On Thu, Dec 27, 2018, 22:32 Dominic Evans ***@***.***> wrote: I hit the same issue. I just manually patched engine.go to print out details of the label collision and ran my queries through that to see which were the cause — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#4608 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAICBsRRFl27omorrX545sZa46fXwSfVks5u9TyHgaJpZM4WpduA> .

[tariq1890]

I can take this issue up.

[juliusv]

@tariq1890 Please do!