How to speed up the query result time

Question

While executing the following query the result comes after 22 secs.
How could i minimize this time ? Please suggest.
SELECT DISTINCT
       PERIOD_NO,
       ITEM_CODE,
       ITEM_NAME,
       SUM(CASE WHEN LEVEL_TYPE IN 'SUB ZONE' THEN WD_FORECAST_QTY_BU / CNV / ML ELSE 0 END)TOT_FRCST
FROM   SHEET_HEAD, SHEET_DETAIL, ITEM_DETAIL
WHERE  WH_SYS_ID = WD_WH_SYS_ID
AND    ITEM_CODE = WD_ITEM_CODE(+)
AND    PERIOD_NO BETWEEN :PERIOD1 AND :PERIOD2
AND    WH_REVISION_NO = 0
AND    WH_LEVEL_CODE NOT IN (SELECT WH_LEVEL_CODE FROM SHEET_HEAD WHERE WH_REVISION_NO = 1)
GROUP BY   ITEM_CODE, ITEM_NAME, WH_PERIOD_NOSanjay

Answer 1

Hi,
with speed the suggest would always be expirement with indexing on the parameters, after first analysing what is currently happening in terms of full table scans etc that are 'most expensive'.
In terms of the specifics on your code the following sticks out as costly; -
AND    WH_LEVEL_CODE NOT IN (SELECT WH_LEVEL_CODE FROM SHEET_HEAD WHERE WH_REVISION_NO = 1)Here I would see if you can rewrite it, perhaps with a left join / self join to achieve the same ends, compare the timings you may be surprised.
regards,
Robert.