MySQL optimized slow log query

One 、 Slow query log (slow_query_log) Concept

about SQL And index optimization , We will use explain To analyze SQL sentence . But there are thousands of real enterprise projects SQL, We can't start from scratch, one by one explain To analyze . Where can we get those long running , Performance consuming SQL？？

When we analyze the business involved in the project sql Efficiency time , You should open the slow query log , According to specific business 、 Specific concurrency , To estimate a time limit , The entire query cannot exceed the upper limit , Start the business after setting , During the pressure test, we will find out what sql Our query exceeded our forecast time , So these sql Will be recorded in the slow query log , And then use explain Analyze these time-consuming statements , Judge why it is inefficient . You can know whether the index is useful , Or there is no index to be added ( Primary key 、unique Or secondary index ), Or the index is used , However, due to the large amount of data in the table , It takes a long time , At this time, we can divide the table into several small tables , Let's see if there is grouping and sorting , If there is where There is also grouping and sorting , Consider adding a union index .

Slow query log related parameters ：show variables like '%slow_query%';（MySQL Many global switches defined , Are stored in global variables , It can be used show/set variables View or set the value of global variable ）

The slow query log switch is off by default
Slow query log path ： Default in /var/lib/mysql/ Next

The slow query log contains all the execution time exceeding the parameter long_query_time( Company ： second ) Of the set value SQL Statement log , stay MySQL Use the command to view , as follows ：

This value can be modified , as follows ：

Now change it to more than 1 Of a second SQL Will be recorded in the slow query log ！ It can be set to 0.01 second , Express 10 millisecond .

Two 、 Slow query log practice

1. Turn on the slow query log switch

When turning on the slow query log switch , Error indication slow_query_log It's a global The variable of （ Others only affect the current session The variable of , Such as ：long_query_time 、profiling）, The modification will affect all session, That is, it affects all users who are accessing the current MySQL server The client of .

Turn on the slow query log switch successfully ！

2. Reasonable setting 、 The upper limit of slow query time acceptable to the business long_query_time

View another session

Discovery is still default 10s, so long_query_time It only affects the present session

3. Pressure measurement performs various businesses

It has exceeded our set long_query_time=0.1s

4. View slow query log

Slow query log path ：/var/lib/mysql/

5. use explain Analyze these time-consuming sql sentence , So as to optimize

It turned out to be a whole table search , Sweep the whole primary key index tree . And then again paswd As a filter , The index should be set , Even if you set the index, you still can't use the index , And then I found out that because passwd String format involves type conversion , So there's no index , Correct sql It should be the following statement .

To sum up the problem at the beginning

Next, we can give some examples , For example, when I do a business, I use where Filter condition plus order by, And then use explain After analysis using filedort, Involving external sorting , Why does it involve external sorting ？ Because our data and indexes are stored on the disk , If you don't have a proper index , We have to do it. order by You can only sort externally , This is time-consuming , Then I tried to use where Filter conditions and order by The sorted fields establish a union index ...

Then it will be combined with the index of various problems mentioned in the previous blog , For example, type forced conversion 、 The filter conditions used MySQL No index is used for any of the functions , Take into account the optimization and so on . Reference answer ：MySQL Index FAQs

3、 ... and 、show profiles see sql Specific running time

MySQL Generally, only two decimal places are displayed

open profiling switch , More detailed time can be displayed

No report error , explain profiling Variables only affect the current session