Created following https://tracker.moodle.org/browse/MDL-45584?focusedCommentId=424154&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-424154 comment.

"I just looked at MongoDB and discovered it's broken for identifiers as it doesn't consider them at all when saving. So I can't even use that as an example of multiple identifiers. But as I said, it depends on the store. MongoDB could have put it as part of the collection or as part of the key in any way. We can't say for sure."

Haven't checked the code, just noting it here so it is not forgotten, it needs investigation.