学步园

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.

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.

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”:

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.

To see how the “WITH clause” is used in ANSI SQL-99 syntax, here is an
excerpt from Jonathan Gennick’s great work “Understanding the WITH
Clause” showing the use of the SQL-99 “WITH clause” to traverse a
recursive bill-of-materials hierarchy