Hints

SQL is a declarative language; it only defines what data to retrieve from the database. It doesn’t describe how the data should be fetched. That, as we know, is the job of the query optimizer, which analyzes several candidate execution plans for a given query, estimates the cost of each of these plans, and selects an efficient plan by choosing the cheapest of the choices considered.

But there may be cases when the selected execution plan is not performing as you have expected and, as part of your query troubleshooting process, you may try to find a better plan yourself. Before doing this, keep in mind that just because your query does not perform as you expected, this does not mean a better plan is always possible. Your plan may be an efficient one, but the query may be an expensive one to perform, or your system may be experiencing performance bottlenecks that are impacting the query’s execution.

However, although the query optimizer does an excellent job...