第一种情况： 执行select，慢查询，然后对表alter，被阻塞，然后其他会话执行select，被阻塞
当最后的select被阻塞时，在
open_table_get_mdl_lock中会去获取锁，先 try_acquire_lock_impl获取，成功返回，不成功向等待队列加个ticket，进入等待
mysqld!open_table_get_mdl_lock(THD*, Open_table_context*, TABLE_LIST*, unsigned int, MDL_ticket**) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_base.cc:2680) mysqld!open_table(THD*, TABLE_LIST*, Open_table_context*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_base.cc:3058) mysqld!open_and_process_table(THD*, LEX*, TABLE_LIST*, unsigned int*, Prelocking_strategy*, bool, Open_table_context*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_base.cc:5018) mysqld!open_tables(THD*, TABLE_LIST**, unsigned int*, unsigned int, Prelocking_strategy*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_base.cc:5805) mysqld!open_tables_for_query(THD*, TABLE_LIST*, unsigned int) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_base.cc:6678) mysqld!Sql_cmd_dml::prepare(THD*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_select.cc:363) mysqld!Sql_cmd_dml::execute(THD*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_select.cc:559) mysqld!mysql_execute_command(THD*, bool) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_parse.cc:4407) mysqld!dispatch_sql_command(THD*, Parser_state*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_parse.cc:4988) mysqld!dispatch_command(THD*, COM_DATA const*, enum_server_command) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_parse.cc:1836) mysqld!do_command(THD*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_parse.cc:1320) mysqld!handle_connection(void*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/conn_handler/connection_handler_per_thread.cc:301) mysqld!pfs_spawn_thread(void*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/storage/perfschema/pfs.cc:2900) libsystem_pthread.dylib!_pthread_start (Unknown Source:0) libsystem_pthread.dylib!thread_start (Unknown Source:0)
第二种场景第一个会话执行慢查询，第二个执行flush tables with read lock,第三个会话继续执行查询 原因是第三个被阻塞在下面的函数
tdc_wait_for_old_version
mysql_mutex_lock(&LOCK_open);
if ((share = get_cached_table_share(db, table_name)) &&
share->has_old_version()) {
struct timespec abstime;
set_timespec(&abstime, wait_timeout);
res = share->wait_for_old_version(thd, &abstime, deadlock_weight);
}
mysql_mutex_unlock(&LOCK_open);
return res;
下面的version（）是在table share中获取的version，第一个select执行后没有变，refresh_version是在函数close_cached_tables中增加的，每次flush tables with read lock,都会调用。所以导致下面的判断认为存在老版本，导致第三个回话进入share->wait_for_old_version 等待中
bool has_old_version() const { return version() != refresh_version; }
等待的堆栈如下：
mysqld!tdc_wait_for_old_version(THD*, char const*, char const*, unsigned long, unsigned int) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_base.cc:2717) mysqld!open_table(THD*, TABLE_LIST*, Open_table_context*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_base.cc:3303) mysqld!open_and_process_table(THD*, LEX*, TABLE_LIST*, unsigned int*, Prelocking_strategy*, bool, Open_table_context*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_base.cc:5018) mysqld!open_tables(THD*, TABLE_LIST**, unsigned int*, unsigned int, Prelocking_strategy*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_base.cc:5805) mysqld!open_tables_for_query(THD*, TABLE_LIST*, unsigned int) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_base.cc:6678) mysqld!Sql_cmd_dml::prepare(THD*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_select.cc:363) mysqld!Sql_cmd_dml::execute(THD*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_select.cc:559) mysqld!mysql_execute_command(THD*, bool) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_parse.cc:4407) mysqld!dispatch_sql_command(THD*, Parser_state*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_parse.cc:4988) mysqld!dispatch_command(THD*, COM_DATA const*, enum_server_command) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_parse.cc:1836) mysqld!do_command(THD*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/sql_parse.cc:1320) mysqld!handle_connection(void*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/sql/conn_handler/connection_handler_per_thread.cc:301) mysqld!pfs_spawn_thread(void*) (/Users/xiaoyu.bai/Downloads/mysql-8.0.23/storage/perfschema/pfs.cc:2900) libsystem_pthread.dylib!_pthread_start (Unknown Source:0) libsystem_pthread.dylib!thread_start (Unknown Source:0)
refresh_version的刷新是不是能放到后面，获取锁成功后在刷，这样就不会阻塞select了。 有兴趣学习源码的加群一起学习啊 QQ： 700072075
