MySQL源码学习系列（二）--面试高频问题:general log、slowlog记录顺序

在实际工作中，我们常常会遇到各种各样的数据库问题，比如查询性能低下、数据不一致等。为了更好地诊断和解决这些问题，MySQL 提供了丰富的日志功能，其中 general log 和慢查询日志（slowlog）是两个非常重要的日志类型。

General log 会记录 MySQL 服务器接收到的所有 SQL 语句，就像一个全面的监控摄像头，不放过任何一个数据库操作。而慢查询日志则专注于记录执行时间超过指定阈值的 SQL 语句，帮助我们定位那些 "拖后腿" 的查询，从而进行针对性的优化。理解这两种日志的记录顺序，不仅有助于我们在生产环境中高效地排查问题，也是面试中经常被问到的高频考点。

准备工作

1.1 环境准备

准备一套MySQL8.0.40的源码编译实例，相关编译环境及基本命令可以参考历史文章准备

MySQL8.0.40编译安装

MySQL源码学习系列（一）-- 环境准备及常用命令

1.2 调整参数

开启慢SQL并设置阈值为1s、开启general log

sql 复制代码

mysql> set global slow_query_log=1;Query OK, 0 rows affected (0.00 sec)
mysql> set global long_query_time=1;Query OK, 0 rows affected (0.00 sec)
mysql> set global  general_log=1;Query OK, 0 rows affected (0.00 sec)
mysql> show global variables like '%slow%';+-----------------------------+-----------------------------------------------------------------+| Variable_name               | Value                                                           |+-----------------------------+-----------------------------------------------------------------+| log_slow_admin_statements   | OFF                                                             || log_slow_extra              | OFF                                                             || log_slow_replica_statements | OFF                                                             || log_slow_slave_statements   | OFF                                                             || slow_launch_time            | 2                                                               || slow_query_log              | ON                                                              || slow_query_log_file         | /usr/local/mysql-8.0.40/mysql-test/var/mysqld.1/mysqld-slow.log |+-----------------------------+-----------------------------------------------------------------+7 rows in set (0.01 sec)
mysql> show global variables like '%long%';+----------------------------------------------------------+----------+| Variable_name                                            | Value    |+----------------------------------------------------------+----------+| long_query_time                                          | 1.000000 || performance_schema_events_stages_history_long_size       | 1000     || performance_schema_events_statements_history_long_size   | 1000     || performance_schema_events_transactions_history_long_size | 1000     || performance_schema_events_waits_history_long_size        | 10000    |+----------------------------------------------------------+----------+5 rows in set (0.01 sec)
mysql> show global variables like '%general%';+------------------+------------------------------------------------------------+| Variable_name    | Value                                                      |+------------------+------------------------------------------------------------+| general_log      | ON                                                         || general_log_file | /usr/local/mysql-8.0.40/mysql-test/var/mysqld.1/mysqld.log |+------------------+------------------------------------------------------------+2 rows in set (0.02 sec)
mysql>

通过GDB调试确定执行顺序

2.1 选择断点

可以选择几个断点来设置，只是确定三个日志的写入顺序可以选如下断点（其他的也可）

日志类型	源码文件	设置断点
General Log	sql/log.cc	general_log_write
慢查询日志	sql/log.cc	write_slow
SQL语句	sql/sql_parse.cc	mysql_execute_command

2.2 设置断点

启动gdb后设置断点

objectivec 复制代码

(gdb) b mysql_execute_commandBreakpoint 1 at 0x5c59b7e80d77: file /usr/local/mysql-8.0.40/sql/sql_parse.cc, line 2946.(gdb) b general_log_writeBreakpoint 2 at 0x5c59b85400c8: file /usr/local/mysql-8.0.40/sql/log.cc, line 1374.(gdb) b write_slowBreakpoint 3 at 0x5c59b853d498: file /usr/local/mysql-8.0.40/sql/log.cc, line 687.(gdb) info b Num     Type           Disp Enb Address            What1       breakpoint     keep y   0x00005c59b7e80d77 in mysql_execute_command(THD*, bool) at /usr/local/mysql-8.0.40/sql/sql_parse.cc:29462       breakpoint     keep y   0x00005c59b85400c8 in Query_logger::general_log_write(THD*, enum_server_command, char const*, unsigned long) at /usr/local/mysql-8.0.40/sql/log.cc:13743       breakpoint     keep y   0x00005c59b853d498 in File_query_log::write_slow(THD*, unsigned long long, unsigned long long, char const*, unsigned long, unsigned long long, unsigned long long, bool, char const*, unsigned long) at /usr/local/mysql-8.0.40/sql/log.cc:687(gdb)

2.3 开始执行SQL调试

在sql里执行一个慢SQL

shell 复制代码

mysql> select sleep(2);

gdb里命中第一个断点：Query_logger::general_log_write

cpp 复制代码

(gdb) cContinuing.[Switching to Thread 0x7ba3a4fff640 (LWP 32375)]Thread 39 "connection" hit Breakpoint 2, Query_logger::general_log_write (this=0x5c59bd33d140 <query_logger>, thd=0x7ba310001200, command=COM_QUERY, query=0x7ba31002fc90 "select sleep(2)", query_length=15) at /usr/local/mysql-8.0.40/sql/log.cc:13741374	                                     const char *query, size_t query_length) {(gdb)

gdb继续命中第二个断点：mysql_execute_command

typescript 复制代码

(gdb) cContinuing.
Thread 39 "connection" hit Breakpoint 1, mysql_execute_command (thd=0x7ba310001200, first_level=true) at /usr/local/mysql-8.0.40/sql/sql_parse.cc:29462946	int mysql_execute_command(THD *thd, bool first_level) {(gdb) p m_query_stringNo symbol "m_query_string" in current context.(gdb) p thd.m_query_string$1 = {str = 0x7ba31002fc90 "select sleep(2)", length = 15}

再继续，命中第三个断点：File_query_log::write_slow

cpp 复制代码

(gdb) cContinuing.
Thread 39 "connection" hit Breakpoint 3, File_query_log::write_slow (this=0x5c59d9ba18a8, thd=0x7ba310001200, current_utime=1743142862100420, query_start_utime=1743142567761813, user_host=0x7ba3a4ffd5c0 "root[root] @ localhost [127.0.0.1]", query_utime=294338607, lock_utime=0, is_command=false, sql_text=0x7ba31002fc90 "select sleep(2)", sql_text_len=15) at /usr/local/mysql-8.0.40/sql/log.cc:687687	                                size_t sql_text_len) {(gdb)