Efficient Indexes in MySQL

https://twindb.com Ovais Tariq & Aleksandr Kuzminsky

Who we are Aleks: ●  TwinDB co-founder ●  Dropbox MySQL SRE ●  ex-Percona consultant

Ovais: ●  TwinDB co-founder ●  Lithium Lead SRE ●  ex-Percona consultant

Agenda

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

How data is organized

B+ Tree

O(log(n))

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

Few Advantages

•  Reduced I/O

•  Reduced Rebalancing

•  Extremely efficient range scans

•  Implicit sorting

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=⎡ logn/logp⎤

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%

Table in MySQL (InnoDB)

CREATETABLE`actor`(`actor_id`SMALLINT(5)UNSIGNEDNOTNULL,`first_name`VARCHAR(45)NOTNULL,`last_name`VARCHAR(45)NOTNULL,`last_update`TIMESTAMPNOTNULL,PRIMARYKEY(`actor_id`),KEY`idx_actor_last_name`(`last_name`))ENGINE=InnoDB;

sakila.actor

PRIMARY

idx_actor_last_name actor_id first_name last_name last_update

1 PENELOPE GUINESS 2006-02-1504:34:33

2 NICK WAHLBERG 2006-02-1504:34:33

3 ED CHASE 2006-02-1504:34:33

4 JENNIFER DAVIS 2006-02-1504:34:33

5 JOHNNY WOOD 2006-02-1504:34:33

... ... ... ...

last_name actor_id

AKROYD 58

AKROYD 92

AKROYD 182

ALLEN 118

ALLEN 145

... ...

How Data is Accessed

Fast if accessing table

and producing result

is simultaneous

Point SELECT SELECT*FROMactorWHEREactor_id=3;

actor_id first_name last_name last_update

1 PENELOPE GUINESS 2006-02-1504:34:33

2 NICK WAHLBERG 2006-02-1504:34:33

3 ED CHASE 2006-02-1504:34:33

4 JENNIFER DAVIS 2006-02-1504:34:33

5 JOHNNY WOOD 2006-02-1504:34:33

... ... ... ...

SELECT by range of keys SELECT*FROMactorWHEREactor_id>3;

actor_id first_name last_name last_update

1 PENELOPE GUINESS 2006-02-1504:34:33

2 NICK WAHLBERG 2006-02-1504:34:33

3 ED CHASE 2006-02-1504:34:33

4 JENNIFER DAVIS 2006-02-1504:34:33

5 JOHNNY WOOD 2006-02-1504:34:33

... ... ... ...

Lookup by secondary key

actor_id first_name last_name last_update

117 RENEE TRACY 2006-02-1504:34:33

118 CUBA ALLEN 2006-02-1504:34:33

119 WARREN JACKMAN 2006-02-1504:34:33

... ... ... ...

145 KIM ALLEN 2006-02-1504:34:33

... ... ... ...

last_name actor_id

AKROYD 58

AKROYD 92

AKROYD 182

ALLEN 118

ALLEN 145

... ...

SELECT*FROMactorWHERElast_name=‘ALLEN’;

Step 1 Step 2

Using index for data access

last_name actor_id

AKROYD 182

ALLEN 118

ALLEN 145

ALLEN 194

ASTAIRE 76

... ...

SELECTCOUNT(*)FROMactorWHERElast_name=‘ALLEN’;

Using index for data access EXPLAINSELECTCOUNT(*)FROMactorWHERElast_name=‘ALLEN’;

***************************1.row***************************id:1select_type:SIMPLEtable:actortype:refpossible_keys:idx_actor_last_namekey:idx_actor_last_namekey_len:137ref:constrows:3Extra:Usingwhere;Usingindex

Covering indexes ALTERTABLEactorADDINDEXidx_last_first(last_name,first_name);SELECTfirst_nameFROMactorWHERElast_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:1select_type:SIMPLEtable:actortype:refpossible_keys:idx_actor_last_name,idx_last_firstkey:idx_last_firstkey_len:137ref:constrows:3Extra:Usingwhere;Usingindex

DISTINCT

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

id:1select_type:SIMPLEtable:actortype:indexpossible_keys:idx_actor_last_name,idx_last_firstkey:idx_actor_last_namekey_len:137ref:NULLrows:200Extra:Usingindex

last_name actor_id

AKROYD 182

ALLEN 118

ALLEN 145

ALLEN 194

ASTAIRE 76

... ...

SELECTDISTINCTlast_nameFROMactor

GROUP BY

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

id:1select_type:SIMPLEtable:actortype:indexpossible_keys:idx_actor_last_name,idx_last_first key:idx_actor_last_namekey_len:137ref:NULLrows:200Extra:Usingindex

last_name actor_id

AKROYD 182

ALLEN 118

ALLEN 145

ALLEN 194

ASTAIRE 76

... ...

SELECTlast_name,COUNT(*)FROMactorGROUPBYlast_name

Loose index scan ALTERTABLEactorADDCOLUMNrankINT;UPDATEactorSETrank=ROUND(100*RAND());ALTERTABLEactorADDINDEXidx_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

... ...

Loose index scan SELECTlast_name,MIN(rank)FROMactorGROUPBYlast_name

last_name rank actor_id

AKROYD 40 58

AKROYD 42 92

AKROYD 95 182

ALLEN 19 194

ALLEN 35 118

... ...

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

id:1select_type:SIMPLEtable:actortype:rangepossible_keys:…,idx_last_rank key:idx_last_rankkey_len:137ref:NULLrows:247Extra:Usingindexforgroup-by

Sorting SELECT*FROMactorWHERElast_name='AKROYD'ORDERBYrank

last_name rank actor_id

AKROYD 40 58

AKROYD 42 92

AKROYD 95 182

ALLEN 19 194

ALLEN 35 118

... ...

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

id:1select_type:SIMPLEtable:actortype:refpossible_keys:…,idx_last_rank key:idx_last_rankkey_len:137ref:constrows:3Extra:Usingwhere

Joining tables SELECTtitle,first_name,last_nameFROMfilmJOINfilm_actorONfilm_actor.film_id=film.film_idJOINactorONactor.actor_id=film_actor.actor_idORDERBYtitle;

Response time: 25ms

Joining tables SELECTtitle,first_name,last_nameFROMfilmFORCEINDEX(`idx_title`)JOINfilm_actorONfilm_actor.film_id=film.film_idJOINactorONactor.actor_id=film_actor.actor_idORDERBYtitle;

Response time: 5ms (5 times faster!)

How to compare efficiency

Q&A

Thank you!