Schema design: is uuid as tag a bad practice?

in the documentation, it says to avoid using IDs for tags. Nevertheless, fields are not indexed, so doing a join on them would be painful.

In my case I have two measurements: actions and reactions, where action has a uuid and reaction has an action_uuid for the corresponding action.
Since I want to join actions with reactions, I was putting the two uuids as tags.
Does it make sense or should I prefer fields?
Thanks for the help

Hello @eloparcol,
It really depends on whether or not your tags or id’s are unbounded. If you have an unbounded number of id’s then it mostly likely is a bad idea. It also depends on how frequently you’re expiring data and how quickly you’re expanding your cardinality. If you’re able to continuously reduce your cardinality by deleting old series, then it could be okay. If your tag id’s aren’t unbounded then you can store your id’s in a tag.
Can you roughly estimate your cardinality?