MySQL + MyISAM table size question

Question

Hello, I have a test table. The test table is as follows:

CREATE TABLE  `mytest` (
  `num1` int(10) unsigned NOT NULL,
  KEY `key1` (`num1`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

I inserted 50 million rows in this table.

When I do show table status the avg_row_length is 7. I was expecting to see 4 since mySQL uses 4 bytes for integers. Does the key have an effect on the avg_row_length? When I look at my .MYD file the size is 334 MB which is exactly what it should be given that the avg_row_length is 7, however I was really expecting to see 190 MB given that I only have an int.

+----------------+--------+---------+------------+----------+----------------+-------------+------------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+----------------+---------+
| Name           | Engine | Version | Row_format | Rows     | Avg_row_length | Data_length | Max_data_length  | Index_length | Data_free | Auto_increment | Create_time         | Update_time         | Check_time | Collation         | Checksum | Create_options | Comment |
+----------------+--------+---------+------------+----------+----------------+-------------+------------------+--------------+-----------+----------------+---------------------+---------------------+------------+-------------------+----------+----------------+---------+
| mytest         | MyISAM |      10 | Fixed      | 50000000 |              7 |   350000000 | 1970324836974591 |    600518656 |         0 |           NULL | 2010-05-22 09:15:06 | 2010-05-22 19:32:53 | NULL       | latin1_swedish_ci |     NULL |                |         |

I have included the output of show table status for mytest table. Sorry about the formatting :D Thanks in advance!

Alessandro Ferrucci

Answer 1

I think the problem is with the pointer size MySQL uses by default.

A citation from MySQL reference:

AVG_ROW_LENGTH

An approximation of the average row length for your table. You need to set this only for large tables with variable-size rows.

When you create a MyISAM table, MySQL uses the product of the MAX_ROWS and AVG_ROW_LENGTH options to decide how big the resulting table is. If you don't specify either option, the maximum size for MyISAM data and index files is 256TB by default. (If your operating system does not support files that large, table sizes are constrained by the file size limit.) If you want to keep down the pointer sizes to make the index smaller and faster and you don't really need big files, you can decrease the default pointer size by setting the myisam_data_pointer_size system variable. (See Section 5.1.4, “Server System Variables”.) If you want all your tables to be able to grow above the default limit and are willing to have your tables slightly slower and larger than necessary, you can increase the default pointer size by setting this variable. Setting the value to 7 allows table sizes up to 65,536TB.

Try to set MAX_ROWS and AVG_ROW_LENGTH yourself on table creation or with ALTER TABLE statement, and see, if that helped.