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DatabaseMetaData dmd = conn.getMetaData();
ResultSet rs = dmd.getSchemas();
List<String> schemas = new ArrayList<String>();
while (rs.next()) {
    schemas.add(rs.getString(1));
}
rs.close();
        
for(String schema : schemas) {
    rs = dmd.getTables(null, schema, "%", null);
    while (rs.next()){
        System.out.println(rs.getObject(2) + ", " + rs.getObject(3) + ", " + rs.getObject(4));
    }
}

SYS, SYSALIASES, SYSTEM TABLE
SYS, SYSCHECKS, SYSTEM TABLE
SYS, SYSCOLPERMS, SYSTEM TABLE
SYS, SYSCOLUMNS, SYSTEM TABLE
SYS, SYSCONGLOMERATES, SYSTEM TABLE
SYS, SYSCONSTRAINTS, SYSTEM TABLE
SYS, SYSDEPENDS, SYSTEM TABLE
SYS, SYSFILES, SYSTEM TABLE
SYS, SYSFOREIGNKEYS, SYSTEM TABLE
SYS, SYSKEYS, SYSTEM TABLE
SYS, SYSPERMS, SYSTEM TABLE
SYS, SYSROLES, SYSTEM TABLE
SYS, SYSROUTINEPERMS, SYSTEM TABLE
SYS, SYSSCHEMAS, SYSTEM TABLE
SYS, SYSSEQUENCES, SYSTEM TABLE
SYS, SYSSTATEMENTS, SYSTEM TABLE
SYS, SYSSTATISTICS, SYSTEM TABLE
SYS, SYSTABLEPERMS, SYSTEM TABLE
SYS, SYSTABLES, SYSTEM TABLE
SYS, SYSTRIGGERS, SYSTEM TABLE
SYS, SYSUSERS, SYSTEM TABLE
SYS, SYSVIEWS, SYSTEM TABLE
SYSIBM, SYSDUMMY1, SYSTEM TABLE

// From https://db.apache.org/derby/docs/10.12/ref/rrefsqlj13733.html
// length is an unsigned integer literal designating the length in bytes. The default length for a CHAR is 1
// one column size is 32 bytes, (1 << 5) or (2 ^ 5)
// one row(8 columns) size 256 bytes, (1 << 8) or (2 ^ 8) 
public static final String TABEL_CREATE = "CREATE TABLE DESERIALIZETEST (COL_A CHAR(32), COL_B CHAR(32), COL_C CHAR(32), COL_D CHAR(32), COL_E CHAR(32), COL_F CHAR(32), COL_G CHAR(32), COL_H CHAR(32))";

SELECT COUNT(COL_A) FROM DESERIALIZETEST

SELECT SUM(LENGTH(COL_A)) AS COL_A_SIZE from DESERIALIZETEST