The [custom-engine.html#custom-engine-api-reference-rnd_pos
rnd_pos()] method behaves in a similar
fashion to the rnd_next() method but takes an
additional parameter:
int ha_foo::rnd_pos(byte * buf, byte *pos)
The *pos parameter contains positioning
information previously recorded using the
position() method.
A storage engine must locate the row specified by the position
and return it through *buf in the internal
MySQL row format.