为什么information_schema.tables.data_free总是8388608？

Question

任何人都知道为什么InnoDB中的information_schema.tables.data_free始终为8388608，无论表中有多少行;

table_schema table_name table_rows data_free engine 

g33v1  appraise 0  8388608  InnoDB 

g33v1  areatype 12403  8388608  InnoDB 

g33v1  atype  581982  8388608  InnoDB 

g33v1  atype2  579700  8388608  InnoDB 

谢谢。

Answer 1

我认为这是临时表的最大尺寸分配用于排序和其他需要硬盘空间的东西。因为同样的东西出现在其他情况下也

mysql> show global variables like '%tmp%'; 
+----------------+-----------------------+ 
| Variable_name | Value     | 
+----------------+-----------------------+ 
| bdb_tmpdir  | /usr/local/mysql/tmp/ | 
| max_tmp_tables | 32     | 
| tmp_table_size | 8388608    | 
| tmpdir   | /usr/local/mysql/tmp | 
+----------------+-----------------------+ 

mysql> show global variables like '%myisam%'; 
+---------------------------------+---------------+ 
| Variable_name     | Value   | 
+---------------------------------+---------------+ 
| myisam_data_pointer_size  | 4    | 
| myisam_max_extra_sort_file_size | 2147483648 | 
| myisam_max_sort_file_size  | 2147483647 | 
| myisam_recover_options   | OFF   | 
| myisam_repair_threads   | 1    | 
| myisam_sort_buffer_size   | 4194304  | 
| myisam_stats_method    | nulls_unequal | 
+---------------------------------+---------------