MySQL8 tmp_ table_ Size and Max_ heap_ table_ size

tmp_table_size Specifies the maximum value of the internal memory temporary table , Each thread is assigned .
（ What actually limits it is tmp_table_size and max_heap_table_size The minimum value of ）
If the memory temporary table exceeds the limit ,MySQL Will automatically convert it to disk based MyISAM surface , Store in designated tmpdir Under the table of contents , Default

mysql> SHOW VARIABLES LIKE 'tmpdir';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| tmpdir        | /tmp  |
+---------------+-------+
1 row in set (0.01 sec)