primary key vs covered index

[marcin kot]

hi, why mysql 5.7 using primary key instead of covering index ?

table:
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id_dok | int(11) | NO | PRI | NULL | |
| lp | int(11) | NO | PRI | NULL | |
| naz | varchar(20) | NO | | NULL | |
| rozm | int(8) | NO | | NULL | |
| opis | varchar(50) | YES | MUL | NULL | |
| oso_dod | int(6) | NO | | NULL | |
| dat_dod | datetime | NO | | NULL | |
| mimetype | varchar(20) | NO | | NULL | |
| plik | mediumblob | NO | | NULL | |
+----------+-------------+------+-----+---------+-------+

indexes:
Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| dokumenty | 0 | PRIMARY | 1 | id_dok | A | 2088 | NULL | NULL | | BTREE | | |
| dokumenty | 0 | PRIMARY | 2 | lp | A | 2368 | NULL | NULL | | BTREE | | |
| dokumenty | 1 | dokumenty_multi_i | 1 | opis | A | 134 | NULL | NULL | YES | BTREE | | |
| dokumenty | 1 | dokumenty_multi_i | 2 | naz | A | 1534 | NULL | NULL | | BTREE | | |
| dokumenty | 1 | dokumenty_multi_i | 3 | dat_dod | A | 1867 | NULL | NULL | | BTREE | | |
| dokumenty | 1 | dokumenty_multi_i | 4 | oso_dod | A | 1867 | NULL | NULL | | BTREE | | |
| dokumenty | 1 | dokumenty_multi_i2 | 1 | id_dok | A | 1928 | NULL | NULL | | BTREE | | |
| dokumenty | 1 | dokumenty_multi_i2 | 2 | lp | A | 2180 | NULL | NULL | | BTREE | | |
| dokumenty | 1 | dokumenty_multi_i2 | 3 | opis | A | 2180 | NULL | NULL | YES | BTREE | | |
| dokumenty | 1 | dokumenty_multi_i2 | 4 | naz | A | 2180 | NULL | NULL | | BTREE | | |
| dokumenty | 1 | dokumenty_multi_i2 | 5 | dat_dod | A | 2180 | NULL | NULL | | BTREE | | |
| dokumenty | 1 | dokumenty_multi_i2 | 6 | oso_dod | A | 2187 | NULL | NULL | | BTREE | | |

query:
explain SELECT lp,opis,naz ,dat_dod,oso_dod FROM dokumenty WHERE id_dok=37399;

+----+-------------+-----------+------------+------+----------------------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+----------------------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | dokumenty | NULL | ref | PRIMARY,dokumenty_multi_i2 | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-----------+------------+------+----------------------------+---------+---------+-------+------+----------+-------+