ORA-12954

I went to Techfest 2019 in Brighton. While I was there I saw a presentation by somebody from Pythian. He said that Oracle Database Express Edition is now very similar to Enterprise Edition, it just has certain resource limits applied. This makes it ideal for testing out new features. One of the resource limits is on database size, which is limited to 12 gigabytes. I wondered what would happen if I tried to exceed this. 

I checked the name of the datafile for the USERS tablespace:

SQL> l
  1  select file_name from dba_data_files
  2* where tablespace_name = 'USERS'
SQL> /

FILE_NAME
---------------------------------------------------
C:\APP\ADMIN\PRODUCT\18.0.0\ORADATA\XE\USERS01.DBF

SQL>

I checked its size:

SQL> select bytes from dba_data_files
  2  where tablespace_name = 'USERS'
  3  /

     BYTES
----------
   5242880

SQL>

...then I tried to resize it:

SQL> l
  1  alter database datafile
  2  'C:\APP\ADMIN\PRODUCT\18.0.0\ORADATA\XE\USERS01.DBF'
  3* resize 10g
SQL> /

Database altered.

SQL>

As soon as it looked like the total amount of data would exceed 12 gigabytes, Oracle returned an ORA-12954:

SQL> l
  1  alter database datafile
  2  'C:\APP\ADMIN\PRODUCT\18.0.0\ORADATA\XE\USERS01.DBF'
  3* resize 11g
SQL> /
alter database datafile
*
ERROR at line 1:
ORA-12954: The request exceeds the maximum allowed database size of 12 GB.

SQL>

It is not obvious how this calculation was done but I assume some allowance was made for data in the SYSAUX, UNDOTBS1 or TEMP tablespaces.