理解MySQL索引与优化

mysql explain select actor_id from actor order by actor_id \G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: actor type: index possible_keys: NULL key: PRIMARY key_len: 4 ref: NULL rows: 4 E

mysql> explain select actor_id from actor order by actor_id \G

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: actor

type: index

possible_keys: NULL

key: PRIMARY

key_len: 4

ref: NULL

rows: 4

Extra: Using index

1 row in set (0.00 sec)

mysql> explain select actor_id from actor order by password \G

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: actor

type: ALL

possible_keys: NULL

key: NULL

key_len: NULL

ref: NULL

rows: 4

Extra: Using filesort

1 row in set (0.00 sec)

mysql> explain select actor_id from actor order by name \G

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: actor

type: index

possible_keys: NULL

key: name

key_len: 18

ref: NULL

rows: 4

Extra: Using index

1 row in set (0.00 sec)