Oracle Hints: Purpose and Implementation

Hints in Oracle are directives used to instruct the query optimizer on how to execute the query. They can guide the optimizer in selecting a specific execution plan to enhance query performance.

Specifically, Hint can be used for the following aspects:

1. Forcing plan choice: Hint allows you to specify the use of a specific execution plan instead of the optimizer automatically choosing one. This can be very useful for developers to manually select a better execution plan based on their experience and needs.
2. Change the order of joins: When a query involves multiple tables, Oracle optimizer will automatically choose the join order based on statistics. However, sometimes developers may know that a specific join order would be more efficient, in which case they can use a Hint to specify the join order.
3. Change the type of connection: The optimizer will automatically choose a connection type (such as Nested Loop Join, Hash Join, Merge Join, etc.), but sometimes developers may know which type of connection is more suitable. In this case, you can use a Hint to specify the connection type.
4. Forced index usage: Sometimes the optimizer may choose a full table scan instead of utilizing an index, even though developers may know that using an index would be more efficient. In such cases, you can use a Hint to force the use of the index.

It is important to note that while hints can improve query performance, excessive use of hints can lead to difficulties in maintenance, decreased readability, and potential risks. Therefore, hints should be used cautiously, only when necessary, and after thorough testing and assessment.