Performance tuning on Siebel table

Question

Hi Team,
Below query i am using to get data from s_contact table.
SELECT   PERSON_UID as PARTY_UID,
  PERSON_UID as PERSON_UID,
  RIV_FLG,
  INTEGRATION_ID
FROM SIEBEL.S_CONTACT While checking explain plan, total cost it gives nearly 40,000. If i put /*+ FIRST_ROWS(100) */, total cost it gives 700.
While i use this query along with other query which has cost, 1000, full query gives me cost ~45,000. I do not know why i am facing this issue. The table S_CONTACT has 1.5 M records where as other query fetches only 23,000 records.

Answer 1

Do you actually want to retrieve all the rows from the S_CONTACT table? If so, the /*+ FIRST_ROWS(100) */ hint will be counterproductive-- you don't want Oracle to optimize the retrieval of the first 100 rows, you want it to optimize the retrieval of the last row. If not, your query should alert the optimizer to the fact that you only care about the first 100 rows by doing something like
SELECT <<list of columns>>
  FROM (SELECT <<list of columns>>, row_number() over (order by <<something>>) rnk
           FROM siebel.s_contact)
WHERE rnk <= 100Ignore the cost.
Justin