In addition, you probably
designed and tested your systems in a limited environment, so your idea of the
optimal execution path may not match the reality of the production environment,
especially as it evolves.
??? The optimizer has a dynamically changing view of the database and its data. The
statistics used by the cost-based optimizer can change with each automated collection.
In addition to the changing statistical conditions, the internal workings
of the optimizer are occasionally changed to accommodate changes in the way
the Oracle database operates. Since Oracle9i, the cost-based optimizer takes into
account the speed of the CPU, and since Oracle Database 10g leverages statistics
on I/O. If you force the selection of a particular query plan with a hint, you
might not benefit from changes in Oracle.
??? A bad choice by the optimizer may be a sign that something is amiss in your
database. For the most part, the query optimizer selects the optimal execution
path. What may be seen as a mistake by the query optimizer can, in reality, be
traced to a misconception about the database and its design or to an improper
implementation.
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