The presentation reveals how MySQL/InnoDB organizes primary and secondary indexes and how MySQL uses the indexes to execute user queries

1. Indexes in MySQL
Aleksandr Kuzminsky
https://twindb.com
How to use indexes efficiently

2. Agenda
1. How data is organized
2. Data access

3. Who we are
Aleks:
● TwinDB co-founder
● Dropbox DBA
● ex-Percona consultant
Ovais:
● TwinDB co-founder
● Lithium lead DBA
● ex-Percona consultant

4. How data is organized

5. Table in MySQL (InnoDB)
CREATE TABLE `actor`(
`actor_id` SMALLINT(5) UNSIGNED NOT NULL,
`first_name` VARCHAR(45) NOT NULL,
`last_name` VARCHAR(45) NOT NULL,
`last_update` TIMESTAMP NOT NULL,
PRIMARY KEY (`actor_id`),
KEY `idx_actor_last_name` (`last_name`)
) ENGINE=InnoDB;

6. B+ Tree
● O(log(n))
● Shallow
● Data in leaf pages

7. sakila.actor
PRIMARY idx_actor_last_name
actor_id first_name last_name last_update
1 PENELOPE GUINESS 2006-02-15 04:34:33
2 NICK WAHLBERG 2006-02-15 04:34:33
3 ED CHASE 2006-02-15 04:34:33
4 JENNIFER DAVIS 2006-02-15 04:34:33
5 JOHNNY WOOD 2006-02-15 04:34:33
... ... ... ...
last_name actor_id
AKROYD 58
AKROYD 92
AKROYD 182
ALLEN 118
ALLEN 145
... ...

8. Data Access
Fast if
accessing table
and
producing result
is
simultaneous

9. Point SELECT
SELECT * FROM actor WHERE actor_id = 3;
actor_id first_name last_name last_update
1 PENELOPE GUINESS 2006-02-15 04:34:33
2 NICK WAHLBERG 2006-02-15 04:34:33
3 ED CHASE 2006-02-15 04:34:33
4 JENNIFER DAVIS 2006-02-15 04:34:33
5 JOHNNY WOOD 2006-02-15 04:34:33
... ... ... ...

10. SELECT by range of keys
SELECT * FROM actor WHERE actor_id > 3;
actor_id first_name last_name last_update
1 PENELOPE GUINESS 2006-02-15 04:34:33
2 NICK WAHLBERG 2006-02-15 04:34:33
3 ED CHASE 2006-02-15 04:34:33
4 JENNIFER DAVIS 2006-02-15 04:34:33
5 JOHNNY WOOD 2006-02-15 04:34:33
... ... ... ...

11. Lookup by secondary key
actor_id first_name last_name last_update
117 RENEE TRACY 2006-02-15 04:34:33
118 CUBA ALLEN 2006-02-15 04:34:33
119 WARREN JACKMAN 2006-02-15 04:34:33
... ... ... ...
145 KIM ALLEN 2006-02-15 04:34:33
... ... ... ...
last_name actor_id
AKROYD 58
AKROYD 92
AKROYD 182
ALLEN 118
ALLEN 145
... ...
SELECT * FROM actor WHERE last_name = ‘ALLEN’;
Step 1 Step 2

12. Using index for data access
last_name actor_id
AKROYD 182
ALLEN 118
ALLEN 145
ALLEN 194
ASTAIRE 76
... ...
SELECT COUNT(*) FROM actor WHERE last_name = ‘ALLEN’;

13. Using index for data access
EXPLAIN SELECT COUNT(*) FROM actor WHERE last_name = ‘ALLEN’;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: actor
type: ref
possible_keys: idx_actor_last_name
key: idx_actor_last_name
key_len: 137
ref: const
rows: 3
Extra: Using where; Using index

14. Covering indexes
ALTER TABLE actor ADD INDEX idx_last_first(last_name, first_name);
SELECT first_name FROM actor WHERE last_name = 'ALLEN'
last_name first_name actor_id
AKROYD KIRSTEN 182
ALLEN CUBA 118
ALLEN KIM 145
ALLEN MERYL 194
ASTAIRE ANGELINA 76
... ...
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: actor
type: ref
possible_keys: idx_actor_last_name,
idx_last_first
key: idx_last_first
key_len: 137
ref: const
rows: 3
Extra: Using where; Using index

15. DISTINCT
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: actor
type: index
possible_keys: idx_actor_last_name,
idx_last_first
key: idx_actor_last_name
key_len: 137
ref: NULL
rows: 200
Extra: Using index
last_name actor_id
AKROYD 182
ALLEN 118
ALLEN 145
ALLEN 194
ASTAIRE 76
... ...
SELECT DISTINCT last_name FROM actor

16. GROUP BY
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: actor
type: index
possible_keys: idx_actor_last_name,idx_last_first
key: idx_actor_last_name
key_len: 137
ref: NULL
rows: 200
Extra: Using index
last_name actor_id
AKROYD 182
ALLEN 118
ALLEN 145
ALLEN 194
ASTAIRE 76
... ...
SELECT last_name, COUNT(*) FROM actor GROUP BY last_name

17. Loose index scan
ALTER TABLE actor ADD COLUMN rank INT;
UPDATE actor SET rank = ROUND(100 * RAND());
ALTER TABLE actor ADD INDEX idx_last_rank (last_name, rank);
last_name rank actor_id
AKROYD 40 58
AKROYD 42 92
AKROYD 95 182
ALLEN 19 194
ALLEN 35 118
... ...

18. Loose index scan
SELECT last_name, MIN(rank) FROM actor GROUP BY last_name
last_name rank actor_id
AKROYD 40 58
AKROYD 42 92
AKROYD 95 182
ALLEN 19 194
ALLEN 35 118
... ...
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: actor
type: range
possible_keys: …, idx_last_rank
key: idx_last_rank
key_len: 137
ref: NULL
rows: 247
Extra: Using index for group-by

19. Sorting
SELECT * FROM actor WHERE last_name = 'AKROYD' ORDER BY rank
last_name rank actor_id
AKROYD 40 58
AKROYD 42 92
AKROYD 95 182
ALLEN 19 194
ALLEN 35 118
... ...
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: actor
type: ref
possible_keys: …, idx_last_rank
key: idx_last_rank
key_len: 137
ref: const
rows: 3
Extra: Using where

20. Joining tables
SELECT title, first_name, last_name
FROM film
JOIN film_actor
ON film_actor.film_id = film.film_id
JOIN actor
ON actor.actor_id = film_actor.actor_id
ORDER BY title;

21. Joining tables
SELECT title, first_name, last_name
FROM film FORCE INDEX (`idx_title`)
JOIN film_actor
ON film_actor.film_id = film.film_id
JOIN actor
ON actor.actor_id = film_actor.actor_id
ORDER BY title;

22. How to compare efficiency

23. Q&A
Thank you!