oracle sql update join two tables

when I execute the following query the sql statement return 1 row
SELECT CTC.COMPANYID,
CTC.CUSTOMERID,
CTC.APPOINTOFCAID,
CTC.ACCKINDID,
CTC.TRNDATE,
SUBSTR(CTO.TRNDOCNO, 0, INSTR(CTO.TRNDOCNO, '_')) ODOCNO,
CTC.TRNDOCNO,
CTO.TRNDOCNO OTRNDOCNO,
CTO.PERIODID OPERIODID,
CTO.PROCESSID OPROCESSID,
CTC.PERIODID CPERIODID,
CTC.PROCESSID CPROCESSID
FROM CUSTOMER_TRANSACTIONS_20070716 CTO, CUSTOMER_TRANSACTIONS CTC
WHERE CTO.PERIODID = '2005-2006'
AND CTO.PROCESSID = '200612'
AND CTO.COMPANYID = CTC.COMPANYID
AND CTO.CUSTOMERID = CTC.CUSTOMERID
AND CTO.APPOINTOFCAID = CTC.APPOINTOFCAID
AND CTO.APPOINTOFCAID = 104392
AND CTO.ACCKINDID = CTC.ACCKINDID
AND SUBSTR(CTO.TRNDOCNO, 0, INSTR(CTO.TRNDOCNO, '_')) = SUBSTR(CTC.TRNDOCNO, 0, INSTR(CTC.TRNDOCNO, '_'))
AND CTO.PERIODID <> CTC.PERIODID
AND CTO.PROCESSID <> CTC.PROCESSID

when I execute the following update query

UPDATE CUSTOMER_TRANSACTIONS CTC
SET PERIODID = '2005-2006,  
       PROCESSID  = '200612'
WHERE EXISTS (
  SELECT NULL
    FROM CUSTOMER_TRANSACTIONS_20070716 CTO
    WHERE CTO.PERIODID = '2005-2006'
    AND CTO.PROCESSID = '200612'
    AND CTO.COMPANYID = CTC.COMPANYID
    AND CTO.CUSTOMERID = CTC.CUSTOMERID
    AND CTO.APPOINTOFCAID = CTC.APPOINTOFCAID
      AND CTO.APPOINTOFCAID = 104392
    AND CTO.ACCKINDID = CTC.ACCKINDID
   AND SUBSTR(CTO.TRNDOCNO, 0, INSTR(CTO.TRNDOCNO, '_')) = SUBSTR(CTC.TRNDOCNO, 0, INSTR(CTC.TRNDOCNO, '_'))
   AND CTO.PERIODID <> CTC.PERIODID
   AND CTO.PROCESSID <> CTC.PROCESSID
);

that run for long time and not end after 20'. This is  normal?

queries probably has different execution plans. if you have computed statitics probably select uses some hash join, update nested loop.
isn't here some locks ?

you can check with adding FOR UPDATE NOWAIT into your select statement. if it fires ora-54 someoe else has locked rows you are going to update