Overview of database files

Many databases consist of a single database file. In this case, the database server uses three files while it is running:

The database file This file holds the database information. It typically has the extension .db.

For information on creating databases, see Working with databases.
The transaction log This file holds a record of the changes made to the database file, and is needed for recovery and replication. It typically has the extension .log.

For information on the transaction log, see Backup and Data Recovery.
The temporary file This file is used by the database server to hold information needed during a session. It is not needed once the database server shuts down. The file has a server-generated name with the extension .tmp. It is held in the system temporary directory.

The temporary file is created and maintained by the server. You do not need to do anything with the temporary file.

Other files can also be used as part of a database system:

Additional database files You can spread your data over several files, each on held in a separate file. These additional files are called dbspaces.
Transaction log mirror files For additional security, you can create a mirror copy of the transaction log. This file typically has the extension .mlg.
Write files If the database file is a read-only file (for example, if it is distributed on CD-ROM) an additional file named a write file can be used, which holds changes made to the data.
Compressed database files You can compress a database file. The resulting file is read only, but can be used in conjunction with a write file.
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This chapter describes how to create, name, and delete the different kinds of files involved in a database system.