Let's say I have 16 million records on one collection and 48 million records on another collection. All 16 million records are subset of 48 million, what would be the best way of joining these two collections?

Collection A - "ID" | "Column1" | "Column2"
Collection B - "ID" | "Column2" | "Column3"

Reultant Collection : "ID" | "Column1" | "Column2" | "Column3"

Can some one share the most efficient way?

Thanks,
sid