All the tables referenced by the query are updatable.

The optimizer does not consider materialized views for a SELECT statement that is inherently updatable, or is explicitly declared in an updatable cursor. This situation can occur when using Interactive SQL, which utilizes updatable cursors by default for SELECT statements.

The statement is a simple DML statement that uses optimizer bypass and is optimized heuristically. However, you can force cost-based optimization of any SELECT statement using the FORCE OPTIMIZATION option of the OPTION clause. See SELECT statement.

The query's execution plan has been cached, as in the case of queries contained inside stored procedures and user-defined functions. The database server may cache the execution plans for these queries so that they can be reused. For this class of queries, the query execution plan is cached after execution. The next time the query is executed, the plan is retrieved and all the phases up to the execution phase are skipped. See Plan caching.