why nodebb database has objectid and incremental id together?

[noob111]

Isnt it waste of memory that it has both of them?
Can't objectid (which is default by mongodb) be deleted since it already has unique ids like uid, pid, tid and so on?

[PitaJ]

Pretty sure objectid is built into mongo and you can't remove it. I'm no expert though.

[noob111]

I tried but objectid could be replaced if _id was replaced with any type like integer which is same type as incremental ids.