RE: Determination of "deterministic"
- From: John Hallas <John.Hallas@xxxxxxxxxxxxxxxxxx>
- To: "yong321@xxxxxxxxx" <yong321@xxxxxxxxx>, "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
- Date: Sat, 30 Oct 2010 08:46:17 +0100
Co-incidentally we had Julian Dyke on site today giving a seminar about
execution plans and he made the same point about the deterministic keyword not
actually performing any verification. Ric obviously makes a valid point that
you would need unlimited resources to be able to correctly determine whether a
function was deterministic or not but it does make you wonder what the
'function' of the keyword is, perhaps it is only to be used in place of a
comment.
John
www.jhdba.wordpress.com
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of Yong Huang
Sent: 29 October 2010 20:24
To: oracle-l@xxxxxxxxxxxxx
Subject: Determination of "deterministic"
A function can be marked as deterministic if you know the function always
returns the same set of data when you pass the same values to its
arguments (if any). If a function has certain keywords such as rownum
or sysdate, it won't be deterministic even if you use the keyword
"deterministic" in its definition:
SQL> create or replace function f return date
2 deterministic
3 as
4 d date;
5 begin
6 select sysdate into d from dual;
7 return d;
8 end;
9 /
Function created.
SQL> select f from dual;
F
-----------------
20101028 22:27:07
SQL> select f from dual;
F
-----------------
20101028 22:27:08
Thus, presence of "deterministic" is NOT related to the true meaning of
the word "deterministic".
On the other hand, this query
select * from (select * from t order by id) where rownum <= 10;
is definitely deterministic from the user's perspective if id is unique,
even though it has the word "rownum".
Is there a way for Oracle, or software in general, to determine whether
a query is really deterministic, not by checking presence of certain
keywords, but by some other means?
Yong Huang
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