MySQL optimization slow log query

Article Directory

• • I. Concept of slow query log
  • Second, slow query log practice
  • • 1. Turn on the slow query log switch slow_query_log
    • 2. Set a reasonable and business-acceptable slow query time upper limit long_query_time
    • 3. Execute various business under pressure test
    • 4. View slow query log
    • 5. Use explain to analyze these time-consuming SQL statements for targeted optimization
  • Three, show profiles to view the specific running time of sql

I. Concept of slow query log

For SQL and index optimization problems, we will use explain to analyze the SQL statement.But a real enterprise-level project has thousands of pieces of SQL, and it is impossible for us to analyze it one by one from scratch.Where can we get those long-running, performance-hungry SQL??

We can open the Slow Query Log:

Estimate a time limit (20ms, 100ms) according to the specific business and concurrency. After setting the business, open the slow query log after the stress test, and you will see the SQL that exceeds the execution time, and then use explain to analyzeThese time-consuming SQL statements

The steps are as follows:

1. Turn on the slow query log switchslow_query_log
2. Set a reasonable and business-acceptable slow query time limit
3. Pressure testing to perform various services
4. View the slow query log to find all time-consuming SQL statements
5. Analyze these time-consuming SQL statements with explain for targeted optimization

MySQL can set a slow query log. When the SQL execution time exceeds the time we set, these SQL will be recorded in the slow query log. Then we can view the log and analyze the execution plan of these SQL with explain.To determine why the efficiency is low, is the index not used?Or is there something wrong with the index itself?Or the index is used, but because the amount of data in the table is too large, it takes a long time, then we can divide the table into multiple small tables and so on.

The parameters related to the slow query log are as follows:
(Many global switches defined by MySQL are stored in global variables, which can be viewed or set with show/set variablesvalue of a global variable)

The slow query log switch is disabled by default
The path of the slow query log: the default is under /var/lib/mysql/

The slow query log records the log containing all SQL statements whose execution time exceeds the value set by the parameter long_query_time (unit: second), which can be viewed with the command on MySQL, as follows:

This value can be modified:

Second, slow query log practice

1. Turn on the slow query log switch slow_query_log

When the slow query log switch is turned on, the error message indicates that slow_query_log is a global variable (there are also variables that only affect the current session, such as: long_query_time, profiling). After modification, it will affect all sessions, that is, it will affect all currently accessing the current session.Client of MySQL server.

Open the slow query log switch successfully!

2. Set a reasonable and business-acceptable slow query time upper limit long_query_time

View another session

It is found that it is still the default 10s, so long_query_time only affects the currentsession

3. Execute various business under pressure test

has exceeded our long_query_time=0.1s

p>

4. View slow query log

Path: /var/lib/mysql/

5. Use explain to analyze these time-consuming SQL statements for targeted optimization

Do a search of the entire table, put the primary key index treeScan the whole thing.

We should add an index to password, and then remember that password is in string format, because if it involves type conversion, the index will not be used

Third, show profiles to view the specific running time of sql

MySQL generally only displays the time with two decimal places

Turn on the profiling switch to display more detailed time

No error is reported, indicating that the profiling variable only affects the current session

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