The [custom-engine.html#custom-engine-api-reference-position
position()] method is called after every call
to rnd_next() if the data needs to be
reordered:
void ha_foo::position(const byte *record)
It stores a "position" of a record in
this->ref. The contents of this "position"
is up to you, whatever value you provide will be returned in a
later call to retrieve the row. The only rule - "position" or a
row must contain enough information to allow you later to
retrieve this very row. Most storage engines will store some
form of offset or a primary key value.