1: Enable slow query log and find slow SQL

Why should I start slow query ？

Find the one to optimize sql, Slow query log is mysql A built-in function , You can record the execution of more than a specified time sql sentence .

How to start a slow query

1： Modify the configuration file my.conf,( Need to restart , The configuration is permanently valid , Will not be lost )

lookup my.cnf file

• linux The default path is generally /etc/my.cnf (windows To call down my.ini)
• Add the following to [mysqld] Next service mysqld restart

# Open slow query log 
slow_query_log=ON
# How long it takes will be recorded in the slow query log ( second )
long_query_time=0.5
# Slow query log storage mode , Can support at the same time    File and table storage 
log_output='FILE,TABLE'

• restart mysql The server , Input SHOW VARIABLES LIKE 'slow_query_log' Check whether to turn on ,ON Already open

2： Set... Through global variables ( No need to restart , Once the restart , Configuration lost )

• Execute directly on the client The following command

SHOW VARIABLES LIKE 'slow_query_log'

SET GLOBAL slow_query_log = 'ON';

SET GLOBAL long_query_time = 0.001;

SET GLOBAL log_output = 'FILE,TABLE';

Find slow SQL

Query through table

• When slow_query_log contain TABLE when , Directly through sql Inquire about , without , It may be just opened , The timeout has not been found sql

SELECT * FROM mysql.slow_log

• Meaning of some fields

start_time		 Implement this sql The time of the statement 
user_host			 user name   And the host name 
query_time		 Inquire about sql The time spent in the statement 
lock_time			 Time to acquire lock 
rows_sent			 Number of rows returned 
rows_examined	 The number of rows queried by the server 
db						 Query the database 
sql_text			 Executive sql sentence 
thread_id			 Thread of execution id

• For example, we can sort according to the time-consuming , Find the most time-consuming sql

SELECT
	* 
FROM
	mysql.slow_log
	
ORDER BY query_time DESC

Query by file

• First you need to turn on FILE , Can pass SHOW VARIABLES LIKE 'log_output' Check whether to turn on File store
  
• Query the path of log storage Can pass SHOW VARIABLES LIKE 'slow_query_log_file' Inquire about
  
  
• have access to mysqldumpslow analysis ：
  for example ： Find the most time-consuming front 10 statement

mysqldumpslow -s t -t 10 -a xxx-slow.log

• The specific parameters are as follows ：

* mysqldumpslow --help
Usage: mysqldumpslow [ OPTS... ] [ LOGS... ]

Parse and summarize the MySQL slow query log. Options are

  --verbose    verbose
  --debug      debug
  --help       write this text to standard output

  -v            Show more detailed information 
  -d           debug
  -s ORDER      How to sort , Default at
                al:  Average lock time 
                ar:  Average number of returned records 
                at:  Average query time 
                 c:  Access count 
                 l:  Lock time 
                 r:  Back to the record 
                 t:  Query time 
  -r            take -s In reverse order 
  -t NUM       top n It means , Show the first few 
  -a            Don't show the numbers as N, Display the string as 'S'
  -n NUM       abstract numbers with at least n digits within names
  -g PATTERN    You can write a regular , Only regular guilds show 
  -h HOSTNAME   Slow query log to   Host name -slow.log Format naming ,-h You can specify to read the slow query log of the specified host name , By default *, Read all slow query logs 
  -i NAME      MySQL Server Instance name of （ If used mysql.server startup Script words ）
  -l            Do not subtract the lock time from the total time