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Oracle WITH clause to simplify complex SQL
Oracle Tips by Burleson Consulting |
Using the WITH clause to
simplify complex SQL
Oracle SQL can run faster when complex
subqueries are replaced with global temporary tables. Starting in
Oracle9i release 2, there was an incorporation of a subquery
factoring utility implemented the SQL-99 WITH clause. The WITH
clause is a tool for materializing subqueries to save Oracle from
having to recompute them multiple times.
Also see these notes:
Use of the SQL WITH clause is very similar to
the use of Global Temporary Tables (GTT), a technique that is often
employed to improve query speed for complex subqueries. The
following are some important notes about the Oracle WITH clause:
§
The SQL WITH clause only works on Oracle 9i release 2
and beyond.
§
Formally, the WITH clause was called subquery
factoring.
§
The SQL WITH clause is used when a subquery is
executed multiple times.
§
The ANSI WITH clause is also useful for recursive
queries, but this feature has not yet been implemented in Oracle
SQL.
The following example shows how the Oracle SQL
WITH clause works and see how the WITH clause and Global temporary
tables can be used to speed up Oracle queries.
All Stores with above average sales
To keep it simple, the following example only
references the aggregations once, where the SQL WITH clause is
normally used when an aggregation is referenced multiple times in a
query.
The following is an example of a request to see
the names of all stores with above average sales. For each
store, the average sales must be compared to the average sales for
all stores:
The
relationship between STORE and SALES
Essentially, the query below accesses the STORE
and SALES tables, comparing the sales for each store with the
average sales for all stores. To answer this query, the following
information must be available:
§
The total sales for all stores.
§
The number of stores.
§
The sum of sales for each store.
To answer this in a single SQL statement, inline
views will be employed along with a subquery inside a HAVING clause:
select
store_name,
sum(quantity)
store_sales,
(select sum(quantity) from sales)/(select count(*) from
store) avg_sales
from
store s,
sales sl
where
s.store_key = sl.store_key
having
sum(quantity) > (select sum(quantity) from sales)/(select
count(*) from store)
group by
store_name
;
While this query provides the correct answer, it
is difficult to read and complex to execute as it is recomputing the
sum of sales multiple times.
To prevent the unnecessary re-execution of the
aggregation (sum(sales)), temporary tables could be created and used
to simplify the query. The following steps should be followed:
-
1.
Create a table named T1 to hold the total sales for all
stores.
-
2.
Create a table named T2 to hold the number of stores.
-
3.
Create a table named T3 to hold the store name and the sum of
sales for each store.
A fourth SQL statement that uses tables T1, T2,
and T3 to replicate the output from the original query should then
be written. The final result will look like this:
create table t1 as
select sum(quantity) all_sales from stores;
create table t2 as
select count(*) nbr_stores from stores
create table t3 as
select store_name, sum(quantity) store_sales from store natural
join sales;
select
store_name
from
t1,
t2,
t3
where
store_sales > (all_sales / nbr_stores)
;
While this is a very elegant solution and easy
to understand and has a faster execution time, the SQL-99 WITH
clause can be used instead of temporary tables. The Oracle SQL WITH
clause will compute the aggregation once, give it a name, and allow
it to be referenced, perhaps multiple times, later in the query.
The SQL-99 WITH clause is very confusing at
first because the SQL statement does not begin with the word
SELECT. Instead, the WITH clause is used to start the SQL query,
defining the aggregations, which can then be named in the main query
as if they were real tables:
WITH
subquery_name
AS
(the aggregation SQL statement)
SELECT
(query naming subquery_name);
Retuning to the oversimplified example, the
temporary tables should be replaced with the SQL WITH clause:
WITH
sum_sales AS
select /*+ materialize */
sum(quantity) all_sales from stores
number_stores AS
select /*+ materialize */
count(*) nbr_stores from stores;
sales_by_store AS
select /*+ materialize */
store_name, sum(quantity) store_sales from
store natural join sales
SELECT
store_name
FROM
store,
sum_sales,
number_stores,
sales_by_store
WHERE
store_sales > (all_sales / nbr_stores)
;
Note the use of the Oracle undocumented
materialize hint in the WITH clause. The Oracle
materialize hint is used
to ensure that the Oracle CBO materializes the temporary tables that
are created inside the WITH clause, and its opposite is the
undocumented inline hint. This
is not necessary in Oracle10g, but it helps ensure that the tables
are only created one time.
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Tip! Depending on the release of Oracle
in use, the global temporary tables (GTT) might be a better
solution than the WITH clause because indexes can be created
on the GTT for faster performance. |
SEE CODE DEPOT FOR FULL SCRIPTS
 |
This is an excerpt from my latest book "Oracle
Tuning: The Definitive Reference".
You can buy it direct from the publisher for 50%-off and get
instant access to the code depot of Oracle tuning scripts: |
http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm
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