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Oracle table join order tips

Oracle Database Tips by Donald BurlesonFebruary 8, 2015

 

For manually determining the best table join order (to add an ?ordered? or ?leading? hint), the WHERE clause of the query can be inspected along with the execution plan for the original query.  Also see my notes on how 10g dynamic sampling  influences table join order.

 

The problem with forcing the sequence of table joins with the ordered hint is that it can be impossible to predict a priori the optimal table-join order (the one that has the smallest intermediate baggage).

 

Reducing the size of the intermediate row-sets can greatly improve the speed of the query.

select
  stuff
from
  customer
natural join
  orders
natural join
  item
natural join
  product
where
  credit_rating * extended_credit > .07
and
  (qty_in_stock * velocity) /.075 < 30
or
  (sku_price / 47) * (qty_in_stock / velocity) > 47;


In this example, the four-way table join only returns 18 rows, but the query carries 9,000 rows in intermediate result sets, slowing-down the SQL execution speed.



Sub-optimal intermediate row sets.

If we were somehow able to predict the sizes of the intermediate results, we can re-sequence the table-join order to carry less ?intermediate baggage? during the four-way table join, in this example carrying only 3,000 intermediate rows between the table joins.



Optimal intermediate row sets.

 

Table join order issues:

See my related notes on having the CBO determine the optimal table join order:

 

Table join order in Oracle 8i

In Oracle8i the undocumented parameter "_new_initial_join_orders" was set to FALSE by default, by setting it to TRUE a better determination of join orders is implemented in some cases. Under 9i and 10g this parameter has been defaulted to TRUE. However, resetting any undocumented parameter should not be undertaken lightly, however Oracle itself recommends setting this parameter to TRUE as a result of bug 1002975 in 8.1.6 and 8.1.7.

Our final tip is to set the "_new_initial_join_orders" undocumented parameter to TRUE in 8i. Note that setting event 10131 to level 1 or higher will have the same result if you are squeamish about using undocumented parameters and utilizing the tip to reset optimizer_max_permutations may achieve the same result.  

In 8i and beyond, consider resetting the "_new_initial_join_orders" undocumented parameter to TRUE or set event 10131 to 1 or greater.

 


 

If you like Oracle tuning, you might enjoy my book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


 

 

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