Call now: 252-767-6166  
Oracle Training Oracle Support Development Oracle Apps

 E-mail Us
 Oracle Articles
New Oracle Articles

 Oracle Training
 Oracle Tips

 Oracle Forum
 Class Catalog

 Remote DBA
 Oracle Tuning
 Emergency 911
 RAC Support
 Apps Support
 Oracle Support

 SQL Tuning

 Oracle UNIX
 Oracle Linux
 Remote s
 Remote plans
 Application Server

 Oracle Forms
 Oracle Portal
 App Upgrades
 SQL Server
 Oracle Concepts
 Software Support

 Remote S


 Consulting Staff
 Consulting Prices
 Help Wanted!


 Oracle Posters
 Oracle Books

 Oracle Scripts

Don Burleson Blog 







 hash join hint Tips

Oracle Database Tips by Donald BurlesonMarch 14, 2015

Question:  I want to replace a nested loops join with a hash join, and I need the syntax for a hash join hint.  How do I add a hash join hint to my SQL?

Answer:  The use_hash hint is used to force a hash join in Oracle SQL and you add the use_hash hint as follows:

select /*+ use_hash */
   col1 . . . ;

Please note that you must have enough PGA RAM to enable hash joins.  If yur database does not have enough RAM to allow Oracle to perform a hash join, then the hash join hint will be ignored because Oracle does not have the computing resources.

The use_hash hint requests a hash join against the specified tables. Essentially, a hash join is a technique whereby Oracle loads the rows from the driving table (the smallest table, first after the where clause) into a RAM area defined by the hash_area_size session parameter, but the PGA areas regions are specific to your release.  In 12c the parameter is memory_target if you are using automatic memory management.

 Oracle then uses a hashing technique to locate the rows in the larger second table. As I mention in Chapter 10, a hash join is often combined with parallel query in cases where both tables are very large.

The following query is an example of a query that has been hinted to force a hash join with parallel query:

select /*+ use_hash(e,b) parallel(e, 4) parallel(b, 4) */
emp e,
bonus b
e.ename = b.ename

Here is the execution plan for the hash join. Note that both tables in this join are using parallel query to obtain their rows:

------------------------------ ---------------------------- ----------

Hash joins are often faster than nested loops joins, especially in cases where the driving table is filtered into a small number of rows in the query's where clause.

Enabling Your Database to Accept the use_hash Hint

The use_hash hint is very finicky, and there are many conditions that must be satisfied. It is not uncommon to find that a use_hash hint is ignored and here are some common causes of this problem.

  • Check initialization parameters Make sure that you have the proper settings for optimizer_index_cost_adj, hash_multiblock_io_count, optimizer_max_permutations, and hash_area_size. You can see Chapter 16 for details on setting these parameters.

  • Verify driving table Make sure that the smaller table is the driving table (the first table in the from clause). This is because a hash join builds the memory array using the driving table.

  • Analyze CBO statistics Check that tables and/or columns of the join tables are appropriately analyzed.

  • Check for skewed columns Histograms are recommended only for non-uniform column distributions. If necessary, you can override the join order chosen by the cost-based optimizer using the ordered hint.

  • Check RAM region Ensure that hash_area_size is large enough to hold the smaller table in memory. Otherwise, Oracle must write to the TEMP tablespace, slowing down the hash join.

Oracle Training from Don Burleson 

The best on site "Oracle training classes" are just a phone call away! You can get personalized Oracle training by Donald Burleson, right at your shop!

Oracle training



Burleson is the American Team

Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

Verify experience! Anyone considering using the services of an Oracle support expert should independently investigate their credentials and experience, and not rely on advertisements and self-proclaimed expertise. All legitimate Oracle experts publish their Oracle qualifications.

Errata?  Oracle technology is changing and we strive to update our BC Oracle support information.  If you find an error or have a suggestion for improving our content, we would appreciate your feedback.  Just  e-mail:  

and include the URL for the page.


Burleson Consulting

The Oracle of Database Support

Oracle Performance Tuning

Remote DBA Services


Copyright © 1996 -  2020

All rights reserved by Burleson

Oracle ® is the registered trademark of Oracle Corporation.



Oracle Training at Sea
oracle dba poster

Follow us on Twitter 
Oracle performance tuning software 
Oracle Linux poster