Is there a way to force Oracle to always use a specified index. The hint does not guarantee this.
Is there another method? Below is an example where I want to use the specified index. However
Oracle uses 2 other indices. The ones chosen by Oracle and the query never completes. If I delete
those 2 indices Oracle has no choice but to use the ones specified and query runs fine. The tables/indices are analyzed. The hints are syntactically correct.
Any ideas?
SELECT /*+ INDEX (mbr3 mrts_batches_received_idx_
mbr.batch_received_date, mbr.billing_area_id,
mbr.charts_total, mbr.check_in_out, mbr.record_type, mbr.dept_id
FROM mrts.mrts_batches_received
WHERE rba3.billing_area_id = mbr.billing_area_id
AND (mbr.record_type, mbr.billing_area_id, mbr.date_of_service) IN (
SELECT /*+ INDEX (mbr3 mrts_batches_received_idx_
mbr3.record_type, mbr3.billing_area_id, mbr3.date_of_service
FROM mrts.mrts_batches_received
WHERE mbr3.batch_received_date BETWEEN '01-oct-2007' AND '01-nov-2007'
AND mbr3.dept_id = 'C'
AND mbr3.check_in_out = 'IN')
AND (mbr.record_type = 'R'
OR mbr.record_type = 'M')
AND mbr.date_of_service IS NOT NULL
ORDER BY mbr.billing_area_id, date_of_service, record_type DESC;
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If the other indexes are on different columns, you could modify the statement to prevent it using them. For example, applying a function to a column will prevent it using an index, so if, in your example the query was using an index on dept_id then you could prevent this by concatenating null to it, thereby prevening it from using the index:
AND mbr3.dept_id||null = 'C'
Obviously, if the columns are used by the index you wish to use and the one you don't want it to use then this probably wouldn't work as it would prevent both indexes from being used.
Have you tried using alternative hints to change the way in which the sql is executed. E.g. to force the order in which the tables are accessed or to perform nested loops. I seem to recall using these types of hints a lot on Oracle 8i as the optimiser wasn't very advanced and tended to get it wrong more than it got it right!!!
I understand what you are saying but I wouldn't go as far as saying they are useless. In the early days of the Cost Based Optimiser I found myself using hints to tell Oracle to use the old Rule Based Optimiser, or to use Nested Loops (like the old RBO tended to use) as these were usually more efficient than the execution plans that the CBO came up with!
The CBO is now much more advanced and, providing it has the information (in the form of statistics) tends to get it right. Admittedly, there are situations, like yours, where it doesn't and hints can then be useful (or not ;-)
To try to understand why the CBO is choosing a particular plan it would be necessary to see the statistics relating to the tables and indexes used by your query and the query plan it is using.
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by: angelIIIPosted on 2008-04-17 at 06:57:11ID: 21377069
>Is there a way to force Oracle to always use a specified index
no.
the hints are just that: a hint (please, oracle, consider that index)
now, could you change the subquery on the IN() clause into EXISTS() syntax?