> This is incredibly helpful, thanks for taking the time to write it. I
> don't think there is anything like this level of description of how
> expr works in the docs, but I may have missed it.
To be clear here, and to help locate it in the documentation, this isn't
a special behaviour of alert rule expressions, it's a general feature of
PromQL, the query language. You'll see the same issues anywhere you use
PromQL, such as in Grafana dashboards, ad-hoc queries through the
Prometheus web interface, and so on. It's covered in the general PromQL
documentation to some degree, in that they say that these things are
vector matching, but I think the current PromQL documentation doesn't
specifically cover some of the surprises that you can get here (arguably
it's not the right place for it and it would be better covered in some
sort of introduction/tutorial for PromQL, which doesn't currently
exist).
(There are various clever tricks to deal with the label and set union
issues, although I don't think anyone has collected them all in one
place.)
- cks
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