create tables as - specify the column format

CREATE TABLE iab1
AS
SELECT CAST( NULL AS DATE) null_date
, CAST( NULL AS NUMBER(8) ) null_number
, CAST( NULL AS VARCHAR2(30) ) null_vchar
, CAST( dummy AS VARCHAR2(200) ) dummy_vchar
FROM dual

N.B. dummy is varchar2(1)

compression and parallel

The parallel parameter allows queries against that table to ‘go parallel’.
I.e. execute different components of the query using separate processes and processors.

As it suggests, the compress parameter allows data to be compressed.
For insert/update it only works with an 11g optional (cost) extra called advanced compression.
That said, in 10g and above, you can compress the data after it has been inserted using
‘alter table……compress…’
Compression saves a lot of I/O and hence makes many data warehouse queries much quicker.
There is a down side…
You have to rebuild bitmap indexes after converting a table/partition to be compressed
And ( I’ve just leant this today) compressed table are not supported under 10g for stand-by databases.
It does work with 11g.

On balance compression (I believe) is a really, really good thing.