在第二个表 MySQL 上左加入 2 个表时的索引

Question

This may be so easy for an experienced user, but I'm no able to fully understand what indexes are needed for this simple case.

I have a front-end table that can be sorted by any column besides some filters that can be applied for searching purposes:

id | username | email | name | phone | email_verified_at

The rows shown is a result of joining 2 tables (with a one-to-one relationship):

TABLE USERS

id | autoincrement
name | string
username | string | unique
email | string | unique
email_verified_at | timestamp | nullable

INDEXES
primary(id)
unique(name,id)
unique(username)
unique(email)
unique(email_verified_at,email)

TABLE PROFILES

id | autoincrement
user_id | FK(users)
phone | string

INDEXES
primary(id)
index(user_id)
unique(phone,user_id)

The most basic query, ordered by user id looks like

select
  `users`.`id` as `id`,
  `users`.`name` as `name`,
  `users`.`username` as `username`,
  `users`.`email` as `email`,
  `users`.`email_verified_at` as `email_verified_at`,
  `profiles`.`phone` as `phone`
from
  `users`
  left join `profiles` on `users`.`id` = `profiles`.`user_id`
order by
  `id` asc
limit
  11

Ordering by id, name, username, email, or email_verified_at performs a FULL INDEX SCAN in users table which I assume is correct.

But the trouble (or not) comes when I try to order by phone:

select
  `users`.`id` as `id`,
  `users`.`name` as `name`,
  `users`.`username` as `username`,
  `users`.`email` as `email`,
  `users`.`email_verified_at` as `email_verified_at`,
  `profiles`.`phone` as `phone`
from
  `users`
  left join `profiles` on `users`.`id` = `profiles`.`user_id`
order by
  `phone` asc
limit
  11

The first thing I see is that it's doing a FULL TABLE SCAN in users table. In order to avoid that I need to create a UNIQUE index for users table containing:

id, username, email, name

Therefore the query performs a FULL INDEX SCAN over users table. Despite that, I'm seeing that query cost is too high compared to sorting by users table fields.

Is that OK? Am I missing something here? I'm not sure if this is an expected result or it can be improved.

Thanks in advance

Answer 1

profiles needs an index starting with user_id . The order of the columns in an index does matter.

For further discussion, please provide SHOW CREATE TABLE and EXPLAIN FORMAT=JSON SELECT...