# Derby database limitations

Derby databases have a few limitations.

## Indexes

Indexes are not supported for columns defined on CLOB, BLOB, LONG VARCHAR, XML, and user-defined data types.

If the length of the key columns in an index is larger than half the page size of the index, creating an index on those key columns for the table fails. For existing indexes, an insert of new rows for which the key columns are larger than half of the index page size causes the insert to fail.

Avoid creating indexes on long columns. Create indexes on small columns that provide a quick look-up to larger, unwieldy data in the row. You might not see performance improvements if you index long columns. For information about indexes, see Tuning Derby.

## System shutdowns

The system shuts down if the database log cannot allocate more disk space.

A "LogFull" error or some sort of `IOException` occurs in the derby.log file when the system runs out of space. If the system has no more disk space to append to the derby.log file, you might not see the error messages.

Parent topic: A Derby database
Related concepts
The database directory
Creating, dropping, and backing up databases
Database-wide properties