The cost-based optimizer was made parallel-aware in version 7.3. Previously, parallelism was done by just taking the best serial plan and parallelizing it. From 7.3, CBO works out the best parallel plan instead, which typically looks very different to the best serial plan. This intelligence was of course only built into the CBO, not the RBO, in keeping with Oracle's policy on optimizer enhancements. Accordingly, from 7.3 anything that would require parallelism, such as a degree of parallelism on a table, constrains the optimization to CBO despite hints to the contrary.

Why does the degree of parallelism force cost-based optimization?