Oracle Database Discussions

Hi all,
I need the length of a BLOB column. I realized that the function dbms_lob.getlength() does this job. However, I wonder if this function "computes" the length of the BLOB data which is given as a parameter, or "retrieves" it from the metadata the BLOB keeps (if there are such metadata of the BLOBs).
I have to get lengths of BLOB data in all rows of a huge-size table; and I have to consider time issues. Therefore, if a costly computation will be done when I call dbms_lob.getlength() for each row, I will keep the length of the BLOBs as a separate column (like a metadata).
Thanks in advance.