Performance tuning Issue

Question

Hi folks,
I having a problem with performance tuning ... Below is a sample query
SELECT /*+ PARALLEL (K 4) */ DISTINCT ltrim(rtrim(ibc_item)), substr(IBC_BUSINESS_CLASS, 1,1)
FROM AAA K
WHERE ltrim(rtrim(ibc_item)) NOT IN
select /*+ PARALLEL (II 4) */ DISTINCT ltrim(rtrim(THIRD_MAINKEY)) FROM BBB II
WHERE SECOND_MAINKEY = 3
UNION
SELECT /*+ PARALLEL (III 4) */ DISTINCT ltrim(rtrim(BLN_BUSINESS_LINE_NAME)) FROM CCC III
WHERE BLN_BUSINESS_LINE = 3
The above query is having a cost of 460 Million. I tried creating index but oracle is not using index as a FT scan looks better. (I too feel FT scan is the best as 90% of the rows are used in the table)
After using the parallel hint the cost goes to 100 Million ....
Is there any way to decrease the cost ...
Thanks in advance for ur help !

Answer 1

Be aware too Nalla, that the PARALLEL hint will rule out the use of an index if Oracle adheres to it.
This is what I would try:
SELECT /*+ PARALLEL (K 4) */ DISTINCT TRIM(ibc_item), substr(IBC_BUSINESS_CLASS, 1,1)
FROM AAA K
WHERE NOT EXISTS (
  SELECT 1
    FROM BBB II
  WHERE SECOND_MAINKEY = 3
      AND TRIM(THIRD_MAINKEY) = TRIM(K.ibc_item))
  AND NOT EXISTS (
  SELECT 1
    FROM CCC III
  WHERE BLN_BUSINESS_LINE = 3
    AND TRIM(BLN_BUSINESS_LINE_NAME) = TRIM(K.ibc_item))But I don't like this at all: TRIM(K.ibc_item), and you never need to use DISTINCT with NOT IN or NOT EXISTS.
Try this:
SELECT DISTINCT TRIM(ibc_item), substr(IBC_BUSINESS_CLASS, 1,1)
FROM AAA K
WHERE NOT EXISTS (
  SELECT 1
    FROM BBB II
  WHERE SECOND_MAINKEY = 3
      AND TRIM(THIRD_MAINKEY) = K.ibc_item
  AND NOT EXISTS (
  SELECT 1
    FROM CCC III
  WHERE BLN_BUSINESS_LINE = 3
    AND TRIM(BLN_BUSINESS_LINE_NAME) = K.ibc_itemThis may not work though, since you may have whitespaces in K.ibc_item.