How to speed up SQL execution?

The most expensive step in the SQL preparation process is the generation of the execution plan, particularly when dealing with a query with multiple joins. When RDBMs evaluates table join orders, it must consider every possible combination of tables. For example, a six-way table join has 720 (permutations of 6, or 6 * 5 * 4 * 3 * 2 * 1 = 720) possible ways that the tables can be joined together.

In Oracle, optimizer_search_limit and optimizer_max_permutations parameters work together to place an upper limit on the number of permutations the optimizer will consider. But setting these values will not guarantee the best results.

Hints for Your SQL Statement

Hints are instructions that you include in your SQL statement for the optimizer. Using hints, you can specify join orders, types of access path, indexes to be used, and the intended optimization goals. You must place the hints within /*+ <hint> */, and you should place them after the SELECT key word.

The following statement returns the rows as soon as it finds a few:

SELECT /*+ FIRST_ROWS */ distinct customer_name FROM customer

Where as the following query waits until all the rows are retrieved and sorted before returning them to the client:

SELECT /*+ ALL_ROWS */ distinct customer_name FROM customer ORDER BY customer_name

Index Hint

Indexes play a very important role in SQL tuning. Indexes allow the table data to be indexed and organized, which in turn enables faster retrieval. Merely creating an index does not speed up the query execution. You must make sure that the query's execution plan uses the hinted index. In the following query, when the optimizer uses the index hint, it will be forced to use the specified index for the search on last_name:

SELECT /*+ index(last_name_indx) */ distinct author_names FROM authors WHERE  last_name ='SAN%'

When you analyze the execution plan on this query, you will see the optimizer using this index. You can also instruct the optimizer to choose between a subset of indexes using /*+ index( indx1, indx2) */.

http://psoug.org/reference/hints.html