An in-memory database does not use the file system, but the size limits
listed in the table later in this topic still
apply. For some limits, the maximum value is determined by the available main
memory instead of the available disk space and file system limitations.
A database directory contains the following, as shown in the following
figure.
- 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 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 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 .
The following figure shows the files and directories in the database
directories that are used by the software.
An example of a <ph conref="../conrefs.dita#prod/productshortname"></ph> database
directory and file structure
This figure shows the files and directories that might be found in the main directory of a Derby database called Sales: the service.properties file, and the log, seg0, tmp, and jar directories.
imposes relatively
few limitations on the number and size of databases and database objects.
The following table shows some size limitations of
databases
and database objects.
Size limits for <ph conref="../conrefs.dita#prod/productshortname"></ph>
database objects
Type of Object
Limit
Tables in each database
java.lang.Long.MAX_VALUE Some operating systems
impose a limit to the number of files allowed in a single directory.
Indexes in each table
32,767 or storage
Columns in each table
1,012
Number of columns on an index key
16
Rows in each 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, which
results in limiting the size of a table and size of a row in that table.
For a complete list of restrictions on databases
and database objects, see the .