Derby databases have these limitations:
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.
It is generally not recommended to create indexes on long columns. It is best to 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.
A "LogFull" error or some sort of IOException will occur in the derby.log if the system runs out of space. If the system has no more disk space to append to the derby.log, you might not see the error messages.