What is the optimal index for this query? - mysql

Question

I have the following table in MYSQL 8:
create table session
(
ID bigint unsigned auto_increment
primary key,
session_id varchar(255) null,
username varchar(255) null,
session_status varchar(255) null,
session_time int null,
time_created int null,
time_last_updated int null,
time_ended int null,
date_created date null,
);
I'm executing the following statement:
select * from session where username = VALUE and session_id = VALUE order by time_created desc
What is the the optimal index for the table to speed up this query?
The EXPLAIN query tells me I have two potential indexes, which are:
create index username_3
on session (username, time_created);
create index username_session_id_time_created_desc
on session (username, session_id, time_created desc);
I would have thought the index 'username_session_id_time_created_desc' would have been picked, however the EXPLAIN statement says that index 'username_3' is selected instead.
EDIT*
Result of SHOW CREATE TABLE session:
CREATE TABLE `session` (
`ID` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`session_id` varchar(255) COLLATE utf8_bin DEFAULT NULL,
`username` varchar(255) COLLATE utf8_bin DEFAULT NULL,
`session_status` varchar(255) COLLATE utf8_bin DEFAULT NULL,
`session_time` int(11) DEFAULT NULL,
`time_created` int(11) DEFAULT NULL,
`time_last_updated` int(11) DEFAULT NULL,
`time_ended` int(11) DEFAULT NULL,
`date_created` date DEFAULT NULL,
PRIMARY KEY (`ID`),
KEY `username_3` (`username`,`time_created`),
KEY `username_session_id_time_created_desc` (`username`,`session_id`,`time_created`)
) ENGINE=InnoDB AUTO_INCREMENT=76149265 DEFAULT CHARSET=utf8 COLLATE=utf8_bin
Result of EXPLAIN statement:
select_type: SIMPLE
type: ref
possible_keys: username_3,username_session_id_time_created_desc
key: username_3
key_len: 768
ref: const
rows: 1
Extra: Using where

Answer 1

For this query:
select *
from session
where username = %s and session_id = %s
order by time_created desc
The optimal index is (username, session_id, time_created desc). The first two columns can be in either order.

Answer 2

First I thought you had a typo because you wrote
create index username_session_id_time_created_desc on session (username, session_id, time_created desc);
But your create table shows
KEY `username_session_id_time_created_desc` (`username`,`session_id`,`time_created`)
instead of
KEY `username_session_id_time_created_desc` (`username`,`session_id`,`time_created` DESC)
But, I now think it is using username_3 in MySQL 8.0 because of backward_index_scan (I did not read it all)
How you can tell : there is no filesort in your simple EXPLAIN, so it must be the optimizer can use the username_3 index to sort your result set. (If you remove time_created from any index, you will see Using filesort. If you are not using MySQL 8.0, it might be that the sorting can also be done with username_3 index in your version.)
Fiddle in 5.7 shows "key": "username_session_id_time_created_desc", : well sometimes, but not always... it might depends on the index length (field length).
Whereas in MySQL 8.0 it shows "key": "username_3", and "backward_index_scan": true,
With only the 3 column index it shows a lower query cost so why choose the other index?
My guess is that the 2 column index is much shorter, and since the backward index scan is possible, the sorting is cheap, and the optimizer still prefer to have less IO, and little more computing.