Skip navigation links
13.5.5 Restrictions on BDB Tables »
Section Navigation      [Toggle]

13.5.4. Characteristics of BDB Tables

Each BDB table is stored on disk in two files. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table format, and a .db file contains the table data and indexes.

To specify explicitly that you want a BDB table, indicate that with an ENGINE table option: 

CREATE TABLE t (i INT) ENGINE = BDB;

The older term TYPE is supported as a synonym for ENGINE for backward compatibility, but ENGINE is the preferred term from MySQL 4.0.18 on and TYPE is deprecated.

BerkeleyDB is a synonym for BDB in the ENGINE table option.

The BDB storage engine provides transactional tables. The way you use these tables depends on the autocommit mode:

  • If you are running with autocommit enabled (which is the default), changes to BDB tables are committed immediately and cannot be rolled back.

  • If you are running with autocommit disabled, changes do not become permanent until you execute a COMMIT statement. Instead of committing, you can execute ROLLBACK to forget the changes.

    You can start a transaction with the START TRANSACTION or BEGIN statement to suspend autocommit, or with SET autocommit = 0 to disable autocommit explicitly.

For more information about transactions, see Section 12.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.

The BDB storage engine has the following characteristics:

  • BDB tables can have up to 31 indexes per table, 16 columns per index, and a maximum key size of 1024 bytes (500 bytes before MySQL 4.0).

  • MySQL requires a primary key in each BDB table so that each row can be uniquely identified. If you don't create one explicitly by declaring a PRIMARY KEY, MySQL creates and maintains a hidden primary key for you. The hidden key has a length of five bytes and is incremented for each insert attempt. This key does not appear in the output of SHOW CREATE TABLE or DESCRIBE.

  • The primary key is faster than any other index, because it is stored together with the row data. The other indexes are stored as the key data plus the primary key, so it is important to keep the primary key as short as possible to save disk space and get better speed.

    This behavior is similar to that of InnoDB, where shorter primary keys save space not only in the primary index but in secondary indexes as well.

  • If all columns that you access in a BDB table are part of the same index or part of the primary key, MySQL can execute the query without having to access the actual row. In a MyISAM table, this can be done only if the columns are part of the same index.

  • Sequential scanning is slower for BDB tables than for MyISAM tables because the data in BDB tables is stored in B-trees and not in a separate data file.

  • Key values are not prefix- or suffix-compressed like key values in MyISAM tables. In other words, key information takes a little more space in BDB tables compared to MyISAM tables.

  • There are often holes in the BDB table to allow you to insert new rows in the middle of the index tree. This makes BDB tables somewhat larger than MyISAM tables.

  • SELECT COUNT(*) FROM tbl_name is slow for BDB tables, because no row count is maintained in the table.

  • The optimizer needs to know the approximate number of rows in the table. MySQL solves this by counting inserts and maintaining this in a separate segment in each BDB table. If you don't issue a lot of DELETE or ROLLBACK statements, this number should be accurate enough for the MySQL optimizer. However, MySQL stores the number only on close, so it may be incorrect if the server terminates unexpectedly. It should not be fatal even if this number is not 100% correct. You can update the row count by using ANALYZE TABLE or OPTIMIZE TABLE. See Section 12.4.2.1, “ANALYZE TABLE Syntax”, and Section 12.4.2.5, “OPTIMIZE TABLE Syntax”.

  • Internal locking in BDB tables is done at the page level.

  • LOCK TABLES works on BDB tables as with other tables. If you do not use LOCK TABLES, MySQL issues an internal multiple-write lock on the table (a lock that does not block other writers) to ensure that the table is properly locked if another thread issues a table lock.

  • To support transaction rollback, the BDB storage engine maintains log files. For maximum performance, you can use the --bdb-logdir option to place the BDB logs on a different disk than the one where your databases are located.

  • MySQL performs a checkpoint each time a new BDB log file is started, and removes any BDB log files that are not needed for current transactions. You can also use FLUSH LOGS at any time to checkpoint the Berkeley DB tables.

    For disaster recovery, you should use table backups plus MySQL's binary log. See Section 6.2, “Database Backup Methods”.

    Warning

    If you delete old log files that are still in use, BDB is not able to do recovery at all and you may lose data if something goes wrong.

  • Applications must always be prepared to handle cases where any change of a BDB table may cause an automatic rollback and any read may fail with a deadlock error.

  • If you get a full disk with a BDB table, you get an error (probably error 28) and the transaction should roll back. This contrasts with MyISAM and ISAM tables, for which mysqld waits for sufficient free disk space before continuing.