MySQL Reference Manual :: 14.4.6 Restrictions on BDB Tables

Each BDB table stores in the .db file the path to the file as it was created. This was done to be able to detect locks in a multi-user environment that supports symlinks. However, the consequence is that BDB table files cannot be moved from one database directory to another.

When making backups of BDB tables, you must either use mysqldump or else make a backup that includes the files for each BDB table (the .frm and .db files) as well as the BDB log files. The BDB storage engine stores unfinished transactions in its log files and requires them to be present when mysqld starts. The BDB logs are the files in the data directory with names of the form log.XXXXXXXXXX (ten digits).

If a column that allows NULL values has a unique index, only a single NULL value is allowed. This differs from other storage engines.