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Rewrite SQL Subqueries as Outer Joins

Oracle Tips by Burleson Consulting
January 29, 2004

Because SQL is a declarative language, you can write the same query in many forms, each getting the same result but with vastly different execution plans and performance.

Also see my related notes on rewrite an exists subquery using rank and partition analytics.

In this example, we select all books that do not have any sales.  Note that this is a non-correlated sub-query, but it could be re-written in several ways.

   book_key NOT IN (select book_key from sales);

There are serious problems with subqueries that may return NULL values. It is a good idea to discourage the use of the NOT IN clause (which invokes a sub-query) and to prefer NOT EXISTS (which invokes a correlated sub-query), since the query returns no rows if any rows returned by the sub-query contain null values.

   book b
   NOT EXISTS (select book_key from sales s where b.book_key = s.book_key);

Additionally, the MINUS operator can be used to exclude results with better performance.  For instance:

select book_key from book
select book_key from sales;

This query will query BOOK and SALES with the option of using fast full index or full index scans.

Subqueries can often be re-written to use a standard outer join, resulting in faster performance.  As we may know, an outer join uses the plus sign (+) operator to tell the database to return all non-matching rows with NULL values.  Hence we combine the outer join with a NULL test in the WHERE clause to reproduce the result set without using a sub-query.

   book  b,
   sales s
   b.book_key = s.book_key(+)
   s.book_key IS NULL;

This execution plan will also be faster by eliminating the sub-query.

Note: Prior to Oracle10g there was a bug that caused a huge execution difference between EXISTS and IN.  Starting in 10g release 2 and beyond, Oracle \will automatically rewrite an IN subquery to use the EXISTS clause.

If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



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