Confusing 10gR2 Optimizer Behaviour - Any Explanations?

I've got a 2 node 10gR2 (10.2.0.2) RAC DB. Recently I was trying to figure out why a particular query's explain plan was indicating that an index was not being used when it would clearly help. This is a query on a single table (no joins). The database was doing a full table scan over many millions of rows instead of consulting the index. I checked that the index was listed as "usable" and that optimizer statistics were current to within an hour and still the optimizer wouldn't use the index.
I then had an opportunity to restart the database instance. Immediately upon restarting the instance and submitting the exact same query the plan showed that the index was used -- Eureka! Now, what would change during an instance restart that might prompt the optimizer to come up with a new plan. Could it be that the optimizer is caching plans and it had this "bad plan" cached and kept re-using it every time until the instance was restarted?
Insight? Places I could read more?
Thank you for your time.

Probably Bind variable peeking issue !!!
Check following white paper
http://www.oracle.com/technology/products/bi/db/10g/pdf/twp_general_cbo_migration_10gr2_0405.pdf

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Edited by: Samuel K on Feb 24, 2011 3:13 PM
Edited by: Samuel K on Feb 24, 2011 3:14 PM
Edited by: Samuel K on Feb 24, 2011 3:15 PM

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Confusing 10gR2 Optimizer Behaviour - Any Explanations?

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