Oracle Data Files

I was looking at the contents of a tablespace recently – just the extents, not a block dump – with the intention of shrinking a couple of the associated datafiles (don’t ask why – the reason is both stupid and irrelevant). I needed to know just how much I could shrink immediately, and if there were any quick wins in terms of moving objects from near the end of files so even more space could be reclaimed. I was a little surprised to discover that something which I have been doing for over 20 years was not as common knowledge as I thought it was, so I though I would try to let some more people know how to do this.

So, what’s in a tablespace? It’s made up of a number of datafiles (whether in ASM, RAW or on a journalled file-system is irrelevant for this conversation). Within the datafiles are extents. These are logical groupings of blocks – a multiple of the block size – which show where your objects are stored within the datafile.

I wrote this piece of SQL to show where the extents live in 1992. It still works today, regardless of whether your tablespaces are dictionary or locally managed:

 
SELECT tablespace_name, file_id, owner, segment_name, block_id begin,
blocks, block_id+blocks-1 end, bytes/1024 KB, '' free
FROM sys.dba_extents
where tablespace_name = 'USERS'
UNION
SELECT tablespace_name, file_id, '' owner, '' segment_name, block_id begin,
blocks, block_id+blocks-1 end, bytes/1024 KB, 'FREE' free 
FROM sys.dba_free_space 
where tablespace_name = 'USERS' 
ORDER BY 1,2,5

TSPC ID OWNER  SEGMENT_NAME    BEGIN  BLOCKS   END     KB  FREE
USERS 4 SCOTT  DEPT              128       8   135     64
USERS 4 SCOTT  PK_DEPT           136       8   143     64
USERS 4 SCOTT  EMP               144       8   151     64
USERS 4 SCOTT  PK_EMP            152       8   159     64
USERS 4 SCOTT  SALGRADE          160       8   167     64
USERS 4                          168     128   295   1024 FREE
USERS 4 SYS    TABLE_1           296       8   303     64
USERS 4 SYS    TABLE_1           304       8   311     64
USERS 4 SYS    TABLE_1           312       8   319     64
USERS 4 SYS    TABLE_1           320       8   327     64
USERS 4 SYS    TABLE_1           328       8   335     64
USERS 4 SYS    IND_1             336       8   343     64
USERS 4                          344      40   383    320 FREE
USERS 4                          384     128   511   1024 FREE
USERS 4                          512     128   639   1024 FREE
USERS 6 SYS    TABLE_2           128     128   255   1024
USERS 6                          256   12544 12799 100352 FREE
USERS 6 SYS    IND_2           12800     256 13055   2048

So, as we can see from the output above, the tablespace USERS has 2 datafiles; “4” and “6” (you can identify them fully using view DBA_DATA_FILES). We can shrink file 4 immediately by 1024K+1024K+320K = a bit over 2M.

File 6 is a bit more of a problem. There’s an index IND_2 in the way, unhelpfully located at the end of the datafile, which stops us from shrinking the datafile. If you try to resize the file using ALTER DATABASE DATAFIL E ‘+DATA/orcl/…..’ 10M, it will fail with the error ORA-03297: file contains used data beyond requested RESIZE value. In this case, the remedy may be as simple as performing an ALTER INDEX IND_2 REBUILD TABLESPACE USERS ONLINE; This will (probably) relocate the index somewhere lower in the file and allow us to shrink the datafile. Moving tables with an ALTER TABLE … MOVE command may prove more difficult depending upon your throughput due to the extra locks needed at the start and end of the transaction, and extra care needs to be take with any LOB objects involved.