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Don Burleson Blog 







ordered hint tips

Oracle Tips by Burleson

March 19,  2016

Question: What does the "ordered" hint do?

Answer:  Oracle has the ordered hint to join multiple tables in the order that they appear in the FROM clause.  The ordered hint can be a huge performance help when the SQL is joining a large number of tables together (> 5), and you know that the tables should always be joined in a specific order.

Hinting around with the ordered hint

Oracle must spend a great deal of time parsing multiple table joins to determine the optimal order to join the tables. SQL statements with table joins involving seven or more tables can sometimes take more than 30 minutes to parse because Oracle must evaluate all possible table join orders. This can add up to more than 40,000 orders with only eight tables. The ordered hint is commonly used in conjunction with other hints to suggest a proper join order.

Also see my notes on the related ordered_predicates hint.

The ordered hint requests that the tables listed in the FROM clause of a SQL statement be joined in the order specified, with the first table in the FROM clause specifying the driving table. The driving table should be the table that returns the smallest number of rows. Using the ordered hint will bypass the very expensive and time-consuming parsing operation and speed the execution of Oracle SQL.

In Listing A is an example of a complex query that is forced to perform a nested loop join with a parallel query on the emp table. Note that I�ve used the ordered hint to direct Oracle to evaluate the tables in the order they are presented in the WHERE clause.

Listing A:

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

Also see the related leading_hint.

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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.

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