Finding blocks with rows query

[JDoyle]

The following query is used to find the number of blocks that contain rows:
SELECT COUNT(DISTINCTSUBST(ROWID,15,4)||SUBSTR(ROWID,1,8))
FROM owner.tablename.

I have run this and do not understand how the two substring arguments detect occupancy.

Can somebody help me to understand this?
Thanks in advance.

[SANJAY_G]

Below is the ROWID format fro Oracle7

'BBBBBBBB.RRRR.FFFF'

where
BBBBBBBB is the block number,
RRRR is the slot(row) number, and
FFFF is a file number.

So in yur query you are counting the no. of occupied blocks in all the files.

However the ROWID format has been changed in Oracle8

Sanjay