MySQL database engines

MyISAM
Advantage: high speed and low disk space usage. You can check the size of the corresponding files (folders) in the operating system for the disk usage of a database or table, you can also check the SQL statement show table status.
Disadvantage: there is no data integrity mechanism, that is, transactions and Foreign keys are not supported.

InnoDB
Advantage: transaction and Foreign keys are supported, and the data integrity mechanism is complete. You can use show table status to check disk usage of a database or table.
Disadvantage: the speed is too slow and the disk space is too large. All databases are stored in one (usually) or several files, so you cannot understand the occupied space of a database or table through the operating system.

Bdb
Advantage: supports transactions and does not support foreign keys. As a result of transaction support, foreign keys can be indirectly implemented on the database client (possibly the end customer's server, such as PHP, therefore, data integrity is still guaranteed;
Disadvantages: slow speed and high disk usage. You cannot use show table status to query the space occupied by a database or table. You can use the operating system to understand the corresponding folder of the database, or the size of the corresponding file in the Table. However, because bdb tables always generate log files, the actual disk usage should also include log files, therefore, the size of a database or table is always smaller than the actual space occupied by the operating system.