What's in those files in my Derby DB?

If you look inside your Derby database on disk, you'll see a number of files:

-bash-2.05b$ ls db/seg0
c101.dat  c1b1.dat  c260.dat  c31.dat   c3d1.dat  c71.dat   c8f1.dat  ca1.dat
c10.dat   c1c0.dat  c271.dat  c321.dat  c3e1.dat  c81.dat   c901.dat  cb1.dat
c111.dat  c1d1.dat  c281.dat  c331.dat  c3f1.dat  c850.dat  c90.dat   cc0.dat
c121.dat  c1e0.dat  c290.dat  c340.dat  c400.dat  c861.dat  c911.dat  cd1.dat
c130.dat  c1f1.dat  c2a1.dat  c351.dat  c411.dat  c871.dat  c921.dat  ce1.dat
c141.dat  c200.dat  c2b1.dat  c361.dat  c41.dat   c881.dat  c930.dat  cf0.dat
c150.dat  c20.dat   c2c1.dat  c371.dat  c421.dat  c891.dat  c941.dat
c161.dat  c211.dat  c2d0.dat  c380.dat  c430.dat  c8a1.dat  c951.dat
c171.dat  c221.dat  c2e1.dat  c391.dat  c441.dat  c8b1.dat  c960.dat
c180.dat  c230.dat  c2f0.dat  c3a1.dat  c451.dat  c8c1.dat  c971.dat
c191.dat  c241.dat  c300.dat  c3b1.dat  c51.dat   c8d1.dat  c981.dat
c1a1.dat  c251.dat  c311.dat  c3c0.dat  c60.dat   c8e1.dat  c990.dat

What are these files?

The Derby storage layer uses the concept of a conglomerate, which is a collection of related data stored in a single file. Each table in your database is stored in a conglomerate, and each index of each table is stored in its own conglomerate, so you have one file per table-or-index in your database.

Note that sometimes indexes are created explicitly, via CREATE INDEX, and sometimes indexes are created automatically, such as when you declare a UNIQUE constraint or a PRIMARY KEY constraint.

Derby tracks each conglomerate separately, by its assigned conglomerate number. The conglomerate number is used to create the file name by converting it to hex and constructing the file name cHHH.dat, where HHH is the conglomerate number in hex.

Derby records information about each conglomerate in its system catalogs, so you can simply run a query to find out which conglomerate is used by which table:

 select c.conglomeratenumber, c.isindex, t.tablename 
        from sys.sysconglomerates c, 
             sys.systables t 
        where c.tableid=t.tableid;

So, for example, if you're wondering what is being stored in file c850.dat, you can simply convert 850-hex to 2128-decimal, and then run:

 select c.conglomeratenumber, c.isindex, t.tablename 
        from sys.sysconglomerates c, 
             sys.systables t 
       where c.tableid=t.tableid 
         and c.conglomeratenumber=2128;

Or, if you're wondering which files are used by table EMPLOYEES:

 select c.conglomeratenumber, c.isindex, t.tablename 
        from sys.sysconglomerates c, 
             sys.systables t 
       where c.tableid=t.tableid 
         and t.tablename='EMPLOYEES';

Note that about 60 of the files are used for the system catalogs themselves, so every Derby database will have at least 60 files in its storage directory.