MySQL slow query methods

MySQL slow query methods

For troubleshooting and identifying performance bottlenecks, the most common problems are slow queries in MySQL and queries that do not have to use indexes.

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Method 1: I am using this method.

MySQL and later versions Support recording slow SQL statements.

Mysql> show variables like 'long % '; Note: This long_query_time is used to define how many seconds is slower to calculate as "Slow query"
+ ----------------- + ----------- +
| Variable_name | Value |
+ ----------------- + ----------- +
| Long_query_time | 10.000000 |
+ ----------------- + ----------- +
1 row in set (0.00 sec)

Mysql> set long_query_time = 1; Note: I set 1, that is, if the execution time exceeds 1 second, the query is slow.
Query OK, 0 rows affected (0.00 sec)

Mysql> show variables like 'slow % ';
+ --------------------- + --------------- +
| Variable_name | Value |
+ --------------------- + --------------- +
| Slow_launch_time | 2 |
| Slow_query_log | ON | Note: whether to Enable Logging
| Slow_query_log_file |/tmp/slow. log | Note: where to set
+ --------------------- + --------------- +
3 rows in set (0.00 sec)


Mysql> set global slow_query_log = 'on' Note: Enable Logging

Once the slow_query_log variable is set to ON, mysql starts recording immediately.

In/etc/my. cnf, you can set the initial values of the above MYSQL global variables.
Long_query_time = 1
Slow_query_log_file =/tmp/slow. log

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Method 2: mysqldumpslow command

/Path/mysqldumpslow-s c-t 10/tmp/slow-log
This will output 10 SQL statements with the maximum number of records, of which:

• -S indicates the sorting method. c, t, l, and r are sorted by the number of records, time, query time, and number of returned records, ac, at, al, and ar indicate reverse descriptions;
• -T indicates the top n, that is, the number of previous data records returned;
• -G, followed by a regular expression matching mode, which is case insensitive;

For example
/Path/mysqldumpslow-s r-t 10/tmp/slow-log
You can obtain up to 10 queries from the returned record set.
/Path/mysqldumpslow-s t-t 10-g "left join"/tmp/slow-log
Obtain the query statements containing the left join in the first 10 results sorted by time.

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