QUERY taking longer time than usual

Question

Hello Gurus,
The query below used to take 5-10 minutes depending on the resource availability, but this time it is taking 4-5 hrs to complete this transaction.
INSERT /*+ APPEND */ INTO TAG_STAGING
SELECT /*+ INDEX(A,ALL_tags_INDX1) */
DISTINCT TRIM (serial) serial_num,
TRIM (COMPANY_numBER) COMPANY_NUM,
TRIM (PERSON_id) PERSON_id
FROM ALL_tags@DWDB_link a
WHERE serviceS IN (SELECT /*+ INDEX(B,service_CODES_INDX2) */
services
FROM service_CODES b
WHERE srvc_cd = 'R')
AND (ORDERDATE_date BETWEEN TO_DATE ('01-JAN-2007','dd-mon-yyyy')
AND TO_DATE ('31-DEC-2007','dd-mon-yyyy'))
AND ( (TRIM (status_1) IS NULL)
OR (TRIM (status_1) = 'R')
AND (TRIM (status_2) = 'R' OR TRIM (status_2) IS NULL)
TAG_STAGING table is empty with primary key on the three given columns
ALL_tags@DWDB_link table has about 100M rows
Ideally the query should fetch about 4M rows.
Could any one please give me an idea as to how to proceed to quicken the process.
Thanks in advance
Thanks,
TT

Answer 1

First I'd check the explain plan to make sure that it makes sense. Perhaps an index was dropped or perhaps the stats are wrong for some reason.
If the explain plan looks good then I'd trace it and see where the time is being spent.