Function resolve_system_key_attribute 

pub fn resolve_system_key_attribute(
    name: &str,
    access_method: &str,
) -> Result<(), SQLError>

A system column that is a key attribute needs an operator class of the index’s access method for its type. The transaction and command identifiers of xmin, xmax, cmin and cmax have one only for a hash index, and the tuple identifier of ctid has one for a btree and a hash index; the object identifier of tableoid is ordered wherever integers are.