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 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) */
e.ename,
hiredate,
b.comm
from
emp e,
bonus b
where
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:

OPERATION
----------------------------------------------------------------------
OPTIONS OBJECT_NAME POSITION
------------------------------ ---------------------------- ----------
SELECT STATEMENT
3
HASH JOIN
1
PARALLEL_TO_SERIAL
TABLE ACCESS
FULL EMP 1
PARALLEL_TO_PARALLEL
TABLE ACCESS
FULL BONUS 2

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.

   
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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:  

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