Breaking up query to force join order

If all else fails and you can’t get the optimizer to join the tables together in an efficient order you can break the query into multiple queries saving the intermediate results in a global temporary table.  Here is how to break our three table join into two joins – first sales and products, and then the results of that join with customers:

SQL> create global temporary table sales_product_results
  2  (
  3  sale_date date,
  4  customer_number number,
  5  amount number,
  6  product_type varchar2(12),
  7  product_name varchar2(12)
  8  ) on commit preserve rows;

SQL> insert /*+append */
  2  into sales_product_results
  3  select
  4  sale_date,
  5  customer_number,
  6  amount,
  7  product_type,
  8  product_name
  9  from sales, products
 10  where
 11  sales.product_number=products.product_number and
 12  sale_date between
 13    to_date('01/01/2012','MM/DD/YYYY') and
 14    to_date('01/31/2012','MM/DD/YYYY') and
 15  product_type = 'Cheese';

SQL> commit;

SQL> select
  2  sale_date, product_name, customer_name, amount
  3  from sales_product_results spr, customers c
  4  where
  5  spr.customer_number=c.customer_number and
  6  c.customer_state = 'FL';

SALE_DATE PRODUCT_NAME CUSTOMER_NAME         AMOUNT
--------- ------------ ----------------- ----------
02-JAN-12 Chedder      Sunshine State Co        100
03-JAN-12 Chedder      Green Valley Inc         200
04-JAN-12 Feta         Sunshine State Co        300
05-JAN-12 Feta         Green Valley Inc         400

Breaking a query up like this is a very powerful method of tuning.  If you have the ability to modify an application in this way you have total control over how the query is run because you decide which joins are done and in what order.  I’ve seen dramatic run time improvement using this simple technique.

– Bobby

About Bobby

I live in Chandler, Arizona with my wife and three daughters. I work for US Foods, the second largest food distribution company in the United States. I have worked in the Information Technology field since 1989. I have a passion for Oracle database performance tuning because I enjoy challenging technical problems that require an understanding of computer science. I enjoy communicating with people about my work.
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2 Responses to Breaking up query to force join order

  1. Roy says:

    Good start, but in many cases I guess you wuold like to add indexes and gather statistics(!) on the temp tables.

  2. Bobby says:

    Roy,

    You are right. You can gather stats on each temp table, add indexes, and put hints on the statement.

    – Bobby

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