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Oracle driving table tips

Oracle Tips by Burleson Consulting
April 26,  2009

Question:  When tuning Oracle SQL, what is the driving table, and why is it important to know the driving table?  Doesn't the CBO determine the driving table for me?

Answer:  The driving table is the first table joined, the one that will return the smallest number of rows, and hence, less "baggage" to be passed to later joins.

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. 

Oracle also uses the term driving table when talking about the driving table in a hash join.  Make sure that the smaller table is the driving table (the first table in the from clause when using the ordered hint). This is because a hash join builds the memory array using the driving table.  Oracle recommends that the hash_area_size for the driving table be set at 1.6 times the sum of bytes for the driving table, and you can use the hash_area.sql script to set the hash_area_size for your query session.


 

 

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