A Derby database is stored in files that live in a directory of the same name as the database. Database directories typically live in system directories.
A database directory contains the following, as shown in Derby database directories contain files and directories used by the software.:
- log directory
Contains files that make up the database transaction log, used internally for data recovery (not the same thing as the error log).
- seg0 directory
Contains one file for each user table, system table, and index (known as conglomerates).
- service.properties file
A text file with internal configuration information.
- tmp directory
(might not exist.) A temporary directory used by Derby for large sorts and deferred updates and deletes. Sorts are used by a variety of SQL statements. For databases on read-only media, you might need to set a property to change the location of this directory. See "Creating Derby Databases for Read-Only Use".
- jar directory
(might not exist.) A directory in which jar files are stored when you use database class loading.
Read-only database directories can be archived (and compressed, if desired) into jar or zip files. For more information, see Accessing a read-only database in a zip/jar file.
Derby imposes relatively few limitations on the number and size of databases and database objects. The following table shows some size limitations of Derby databases and database objects:
| Type of Object | Limit |
|---|---|
| tables per database | java.lang.Long.MAX_VALUE Some operating systems impose a limit to the number of files allowed in a single directory. |
| indexes per table | 32,767 or storage |
| columns per table | 1,012 |
| number of columns on an index key | 16 |
| rows per table | no limit |
| size of table | no limit Some operating systems impose a limit on the size of a single file. |
| size of row | no limit--rows can span pages. Rows cannot span tables so some operating systems impose a limit on the size of a single file, and therefore limit the size of a table and size of a row in that table. |
For a complete list of restrictions on Derby databases and database objects, see the Derby Reference Manual.