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Don Burleson Blog 








Oracle SQL query rewriting with temporary tables

Oracle Database Tips by Donald Burleson

End users often don't know how difficult a SQL statement might be to code, and a great example is a simple query 'show me all stores with above average sales?.  This would involve only two tables (a STORE and a SALES table), but it's a simple question with a complex answer.  To answer the question "Show me all stores with above average sales?", we must know:

   - How many stores are there?

   - What are the total sales for each store?

   - What are the total sales for all stores?

Here is the ?university? solution, compact and elegant.  Unfortunately, it's hard to understand and also performs very poorly:

 sum(quantity)                       store_sales,
 (select sum(quantity) from sales)/
 (select count(*) from store)          avg_sales
   store  s,
   sales  sl
   s.store_key = sl.store_key
   sum(quantity) > (select sum(quantity) from sales)/(select count(*) from store)
group by store_name;

Oracle SQL query rewriting for high performance

But what other options do we have?  Oracle introduced Global Temporary Tables (GTT) for removing complex subqueries and allowing us to materialize the intermediate data that we need to solve a complex problem with SQL. 

Here is the same solution with Create Table As Select (CTAS).  It's is easier to understand, and most important, it runs far faster than the ?University? solution:

drop table store_sales;
drop table store_cnt;
drop table store_qty;

create global temporary table store_qty
on commit preserve rows
as select sum(quantity) all_sales from sales;

create global temporary table store_cnt
on commit preserve rows
as select count(*) nbr_stores from store;

create global temporary table store_sales
on commit preserve rows
as select store_name, sum(quantity) store_sales
from store natural join sales group by store_name;

select store_name, store_sales, all_sales / nbr_stores avg_sales
from store_qty, store_cnt, store_sales
where store_sales > (all_sales / nbr_stores);

For more on CTAS and GTT technology, see my notes:

Oracle SQL99 features also allow us to create intermediate materializations with the new ?WITH? clause.  This code is also far faster than the original ?Unversity? solution:

(Note:  You may find a faster execution plan by using Global Temporary tables, depending on your release of Oracle):

sum_sales AS
  ( select /*+ materialize */
    sum(quantity) all_sales from sales ),
number_stores AS
  ( select /*+ materialize */
    count(*) nbr_stores from store ),
sales_by_store AS
  ( select /*+ materialize */
  store_name, sum(quantity) store_sales from
  store natural join sales group by store_name)
   store_name, store_sales, all_sales / nbr_stores avg_sales
   store_sales > (all_sales / nbr_stores);


For more details, see my notes:

If you like Oracle tuning, you may enjoy the new book "Oracle Tuning: The Definitive Reference", over 900 pages of BC's favorite tuning tips & 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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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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