About Oracle WITH clause
Starting in Oracle9i release 2 we see
an incorporation of the SQL-99 “WITH
clause”, a tool for materializing subqueries
to save Oracle from having to re-compute
them multiple times.
Also see these important related SQL
tuning tools:
The SQL “WITH clause” is very similar to the
use of Global temporary tables (GTT), a
technique that is often used to improve
query speed for complex subqueries. Here 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” is called
subquery factoring
• The SQL “WITH clause” is used when a
subquery is executed multiple times
• Also useful for recursive queries
(SQL-99, but not Oracle SQL)
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 WITH clause to
simplify complex SQL
We can also use the SQL-99 “WITH clause”
instead of temporary tables. The Oracle SQL
“WITH clause” will compute the aggregation
once, give it a name, and allow us to
reference it (maybe 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, we use
the “WITH clause” to start our 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 our oversimplified example,
let’s replace the temporary tables with the
SQL “WITH clause” (Note: You may
find a faster execution plan by using Global
Temporary tables, depending on your release
of Oracle):
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 cost-based optimizer
materializes the temporary tables that are
created inside the “WITH” clause. This is
not necessary in Oracle10g, but it helps
ensure that the tables are only created one
time.
It should be noted that the “WITH clause”
does not yet fully-functional within Oracle
SQL and it does not yet support the use of
“WITH clause” replacement for “CONNECT BY”
when performing recursive queries.
The SQL-99 “WITH clause” is very
confusing at first because the SQL statement
does not begin with the word SELECT.
Instead, we use the “WITH clause” to start
our 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);
Note this page where we see a
benchmark showing the difference in
performance of complex SQL subqueries
rewritten with intermediate tables.
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