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Why CBO decide to uses INDEX FULL SCAN ?

edited Jun 11, 2014 6:28AM in SQL Performance (MOSC) 10 commentsAnswered
 Hello,
on our database (Oracle 11G 32bits with ASM storagre), when doing a query on 1h with a filter on a field containing IP addresses (30 differents IP) it is very fast (1 or 2 seconds) for 1 hour as it uses INDEX RANGE SCAN (this field is indexed).
But when period is 2h, it will use INDEX FULL SCAN which can take around 15 minutes.

INDEX_RS_ASC hint does help, but why does CBO decide that INDEX FULL SCAN will be faster when it is obviously not the case ?

I will share the plan in next post if it does not make my browser crash.

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