The [custom-engine.html#custom-engine-api-reference-index_read_idx index_read_idx()] method is identical to [custom-engine.html#custom-engine-index-read index_read()] with the exception that index_read_idx() accepts an additional keynr parameter:

int ha_foo::index_read_idx(byte * buf, uint keynr, const byte * key,
                           ulonglong keypart_map,
                           enum ha_rkey_function find_flag)

The keynr parameter specifies the index to be read, as opposed to index_read(), where the index is already set.

As with the index_read() method, the storage engine must return the row that matches the key according to the find_flag and set a cursor for future reads.