Tool it Up! - Session #3 - MySQL

Date: August 30 2013By: Patrick Demers

Query Process

Query Cache• Add some overhead to every query• Will invalidate any cache referencing a table

that changed• Cannot cache query using :

Query Cache• Current Setting [ON]

must specify SQL_NO_CACHE to skip cache• Will speed up repeating query with the SAME

SYNTAX.• Compact with

FLUSH NO_WRITE_TO_BINLOG QUERY CACHE

Example

Variable Type• Smaller is usually better

● less space on disk / memory and CPU cache. Fewer CPU cycles to process

• Simple is good● Use Integer instead of string (collation / char. set)

• Avoid Null Column● Harder to optimize● Use More Space , require special processing.

Variable Type • Current Table size 12gig• Test Table

● Initial : 312meg● After Type change : 297meg (+-

5%)

Variable Type• Size of a Varchar is 65535• Why use Text then

● Stored separately● Allow innodb to store more row in a block● Use text if not retrieved most of the time● Will require an On Disk Temporary table (group by / order

by)● Use varchar for columns retrieved in most query

Index• B-Tree Index

Index• MySQL will let you index the same column

multiple times• Optimizer will check every index available• Size of the index depends on Variable Type• Indexing a null column requires more

processing

Index• Optimize InnoDB Table when there's a lot of

Insert / Delete.• Don’t create index only thinking of your

'current query'• Test your query often. Adding record in a table

might change an index performance.• InnoDB will lock every row it thinks he needs.

No Index will lock the table.

Index• Mysql can use part of an index as long as it's

left to right• Using a range in a where might prevent the

index to be used fully .● where o.date >= '2013-08-01' AND o.date <= '2013-08-30'

• String can be indexed partially (ex: 10 char on a 25 char field)

EXPLAIN• Useful to check a query performance• Give you Info on what you should change on

your query•

Explain Cheat Sheet part 1

http://www.pythian.com/blog/wp-content/uploads/explain-diagram1.pdf

Explain Cheat Sheet part 2

Explain Cheat Sheet part 3

Profile and Session Status• Useful to find where MySQL Spend time• Show you the real work MySQL is doing , more

precise than EXPLAIN

Example

Other• Information_Schema.Columns

● Useful to search all columns in all databases● Sql injection will try to fetch data from there most of the

time.

• Limit● Try not to use big offset. Use a combination of where and

limit when possible

• Try not to use sub-query● The result of a sub-query is copied to a temp. table with

no index, most of the time it should be avoided.

Other• See the optimized Query

● EXPLAIN EXTENDED● SHOW WARNINGS

• Query are not always the problem• Test your Doctrine query

Reminder• Before Optimizing everything, test what your

doing• If you change a Column Type, make sure it will

never get over that value. (skin_id example)• (Paysite) If you change a column type for a

client, change it everywhere to stay constant• Use new Relic Slow Query Report.• Stay Up to date with new feature.

Resource• MySQL Performance Blog

• MySQL Blog

• Percona Webinar

• Percona Youtube channel

• Pythian Varchar vs Text

• MySQL Group By Optimization

• MySQL Order By Optimization

• High Performance MySQL [book]

• Relational Database Index Design and the Optimizers [book]

Thank You !

Comment , Suggestion and Critics are

Welcome :)