The document summarizes how data is organized and accessed efficiently in MySQL databases using indexes. It discusses how B+ trees are used to organize data in indexes to allow efficient lookups, ranges scans, and sorting. It also provides examples of how indexes can improve the performance of common query types like point lookups, range selections, aggregations, sorting, and joins by enabling access plans that utilize the indexes.

1. Efficient
Indexes in MySQL
https://twindb.com
Ovais Tariq & Aleksandr Kuzminsky

2. Who we are
Aleks:
● TwinDB co-founder
● Dropbox MySQL SRE
● ex-Percona consultant
Ovais:
● TwinDB co-founder
● Lithium Lead SRE
● ex-Percona consultant

3. Agenda
1. How is data organized
2. How is data accessed

5. How data is organized

6. B+ Tree
O(log(n))

7. B+Tree Characteristics
• Leaf node contains data
• Doubly linked list of leaf nodes
• Keys stored in sorted order
• All leaf nodes at the same height

8. Few Advantages
• Reduced I/O
• Reduced Rebalancing
• Extremely efficient range scans
• Implicit sorting

9. Index Height
h is the height of the tree
n is the number of rows in a table
p is the branching factor of the tree
p = page size in bytes/key length in bytes
h = ⎡ log n / log p ⎤

10. Index Height
# records Height with INT
Primary Key
Height with
CHAR(36) Primary
Key
Increase in IO
Operations
1,000 1 2 +100%
1,000,000 2 3 +50%
1,000,000,000 3 4 +33%
1,000,000,000,000 4 5 +25%

11. 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;

12. 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
... ...

13. How Data is Accessed
Fast if
accessing table
and
producing result
is
simultaneous

14. 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
... ... ... ...

15. 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
... ... ... ...

16. 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

17. 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’;

18. 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

19. 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

20. 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

21. 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

22. 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
... ...

23. 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

24. 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

25. 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;
Response time: 25ms

26. 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;
Response time: 5ms (5 times faster!)

27. How to compare efficiency

28. Q&A
Thank you!