Very slow full table scan

Hi,

We are using Release: Oracle 12.1.0.2 with Red Hat Linux.

There is an update operation with very slow full table scan.

The session is 4198 minutes active.

The amount of blocks to read in the full table scan is 81598.

The session had started 3 days ago.

Right now ONLY 1957 blocks were read. The problem is: few blocks read (1957) in 3 days.

Its very stranger this behaivor.

The operation is:

UPDATE CRM_APP.TB_CUSTOMER C SET C.UP_DT = ':p_up_dt'

WHERE EXISTS

(

    SELECT * FROM CRM_APP.TB_CUSTOMER_TMP U

    WHERE   U.CUST_ID = C.CUST_ID

    AND     U.ACC_ID  = C.ACC_ID