MySQL 8.4 Release Notes
性能模式提供了一个user_variables_by_thread表，用于显示用户定义的变量。这些变量是在特定会话中定义的，包括一个@字符在名称前；参见11.4 节，“用户定义变量”。The Performance Schema provides a
user_variables_by_thread table that
exposes user-defined variables. These are variables defined
within a specific session and include a @
character preceding the name; see
Section 11.4, “User-Defined Variables”.
user_variables_by_thread表有以下列:The user_variables_by_thread table
has these columns:
THREAD_ID定义变量的会话的线程标识符。The thread identifier of the session in which the variable is defined.
VARIABLE_NAME变量名，不包括前导的
@字符。The variable name, without the leading@character.VARIABLE_VALUE变量值。The variable value.
user_variables_by_thread 表具有以下索引：The user_variables_by_thread table
has these indexes:
主键在 (
THREAD_ID,VARIABLE_NAME) 上Primary key on (THREAD_ID,VARIABLE_NAME)
TRUNCATE TABLE 不允许用于 user_variables_by_thread 表。TRUNCATE TABLE is not permitted
for the user_variables_by_thread
table.