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11g Hash Outer Joins

Oracle 11g New Features Tips by Burleson Consulting
June 27, 2008

Oracle 11g SQL New Features Tips

A new capability of the optimizer is to use a hash algorithm to execute outer joins.  Using this new hash outer join can result in a 50% reduction in logical IO.  It must be kept in mind that an outer join returns the rows that match, plus the rows that do not match from one or both tables.  Using the PUBS schema, the following example shows the HASH OUTER JOIN at work.

Below, a FULL OUTER JOIN is used with the author and book_author tables. This will return all matching rows and the non-matching rows in both the author table, of authors that have no books, and the book table, which are planned books that do not have authors.

  author full outer join book_author using (author_key)
order by 1;

Id  | Operation               |
|   0 | SELECT STATEMENT        |
|   1 |  SORT ORDER BY          |
|   2 |   VIEW                  |
|   4 |     TABLE ACCESS FULL   |
|   5 |     INDEX FAST FULL SCAN|    


Notice the HASH JOIN FULL OUTER step in the execution plan.  Remote DBAs must keep in mind that there is no hint to force the optimizer to use or not to use the HASH OUTER JOIN method.


In this chapter, the enhancement to SQL introduced in 11g have been explored.  First, the PIVOT and UNPIVOT operators are used to shift data in a result set from rows to columns and columns to rows.  Regular Expressions were also covered, which allow Remote DBAs to pattern match both a pattern and a sub pattern.  Additionally, the regexp_count function returns the number of matches found, and the result_cache hint caches a query result set in the SGA for use by multiple sessions.  These provide a significant performance boost to query execution time and resources.  Also in 11g, the new HASH OUTER JOIN optimizer access method can significantly reduce logical IO. As has been exhibited, all of these new SQL features enhance the developer’s capability to efficiently interact with the database.

New features of 11g SQL covered in this chapter were:

  • Pivot and Unpivot

  • Regular expressions

  • Hash outer joins


This is an excerpt from the new book Oracle 11g New Features: Expert Guide to the Important New Features by John Garmany, Steve Karam, Lutz Hartmann, V. J. Jain, Brian Carr.

You can buy it direct from the publisher for 30% off.

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