Compression and my Oracle OCD

I've been poking around with segment compression lately. I started looking into it whilst preparing my presentation for the Scotland OUG meeting, but now I've started playing with it, I can't stop. I want to know a little bit more every time, it's like an obsessive compulsion. I should probably seek help.

I hadn't realized that columns of length less than 6 don't get compressed, (until I read something Geoff Moss had written on the subject), so I just wanted to prove it to myself. So I created a table with one column, VARCHAR2(5), and populated it with 4194304 rows each containing 'XXXXX'.

SQL> alter table test1 move;

Table altered.

SQL> select bytes/1048576 meg, blocks from dba_segments where segment_name='TEST1';

       MEG     BLOCKS
---------- ----------
  49.84375       6380

SQL> alter table test1 move compress;

Table altered.

SQL> select bytes/1048576 meg, blocks from dba_segments where segment_name='TEST1';

       MEG     BLOCKS
---------- ----------
 44.859375       5742

Eh? What? I though the columns weren't supposed to be compressed?
Why's it smaller? What happened?
A fraction of a second later I remembered that altering a table "compress" also has the effect of setting pct_free to zero (it had previously been 10). And yes, the original 6380 blocks are now 638 fewer. The attempted compression did not compress anything.

Pretty obvious, but note that altering a table back to nocompress still leaves pct_free as zero. I mean, it would, wouldn't it.

When I did the same thing with a VARCHAR2(6) I did indeed get the compression I expected (not fantastic - the longer the column the more benefit, so I expected a poor compression ratio). My table went from 6266 blocks to 5758.

Now I want to see what the reason is for the 6 byte limit. Logically it must simply be that the reference to the value in the symbol table becomes longer that the original data, but given my Oracle Obsessive Compulsive Disorder I am now have to poke about with dumping blocks and look at the internal structure of the symbol table, compressed row data and so on, and so on. Not sure how I'll get on with that, but I'll keep you posted.

My apologies to anyone who stumbled upon this post looking for information about Oracle Configurator Developer, by the way.