Re: Materialize hint
- From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>
- To: "oracle-l" <oracle-l@xxxxxxxxxxxxx>
- Date: Mon, 27 Dec 2004 17:41:35 -0000
In reverse order, the "with" clause names
and defines a subquery before its use in
a query - a bit like a macro in C.
Unlike C macros though, the optimizer can
choose to write your subquery in-line and then
optimise the expanded statement, or create a
temporary table from the definition and use the
temporary table for the main query.
You can choose to use the 'with' clause simply
to make a complex SQL statement tidier, knowing
that there should be no performance benefit in
creating a temporary table.
If you want to control the optimiser, then the
'materialize' hint makes it create a temporary
table; the 'inline' hint makes it perform 'macro-
substitution'.
As far as I know, neither hint is documented.
The 'with' clause (known as subquery factoring)
is quite flexible - though not yet as flexible as
DB2's which can cope with recursive definitions).
Here's an example I wrote to answer a fun puzzle
that Daniel Morgan put out on cdo.server some
months ago.
with age_list as (
select rownum age
from all_objects
where rownum <= 36
),
product_check as (
select
age1.age as youngest,
age2.age as middle,
age3.age as oldest,
age1.age + age2.age +age3.age as summed
from
age_list age1,
age_list age2,
age_list age3
where
age2.age >= age1.age
and age3.age >= age2.age
and age1.age * age2.age * age3.age = (
select max(age) from age_list
)
),
summed_check as (
select
youngest, middle, oldest, summed
from
(
select
youngest, middle, oldest, summed,
count(*) over(partition by summed) ct
from product_check
)
where ct > 1
)
select
*
from summed_check
where
oldest > middle
;
Regards
Jonathan Lewis
http://www.jlcomp.demon.co.uk/faq/ind_faq.html
The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/seminar.html
Public Appearances - schedule updated Dec 23rd 2004
----- Original Message -----
From: "Mladen Gogala" <gogala@xxxxxxxxxxxxx>
To: "oracle-l" <oracle-l@xxxxxxxxxxxxx>
Sent: Saturday, December 25, 2004 5:05 AM
Subject: Materialize hint
I recently ran accross a SQL by Jonathan that uses "materialize" hint.
As was unable to find the hint documented anywhere, and God knows I
tried before asking the question, I must ask the folowing two =20
questions:
1) What does "materialize" hint do and where is it documented?
I was unable to find it documented in either 10g documentation
or 9.2 documentation.
2) The same question for "with generator" clause.
This question is, of course, meant for Jonathan but I'd appreciate
anybody else's answer as well.
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