Why do we need query rewrite enabled for a function-based index?

Question

Oracle 9i
========
I have searched a few sites but could not find any content on it. The question is why do we need to implement query rewrite enabled when we are trying out a function-based index?
Thanks in advance.

Answer 1

You don't, that's a legacy requirement from the early days of function based indexes in Oracle 8i. Here's a quick example running under 9.2.0.6
drop table t1;
create table t1 as
select
from
     all_objects
where
     rownum <= 30000
create or replace function pl_func(i_vc     varchar2)
return varchar2
deterministic
as
begin
     return soundex(i_vc);
end;
-- set the worst case scenario
alter session set query_rewrite_enabled = false;
alter session set query_rewrite_integrity = enforced;
create index t1_i1 on t1(pl_func(object_name));
execute dbms_stats.gather_table_stats(user, 't1')
set autotrace traceonly explain
select
     object_name
from t1
where pl_func(object_name) = 'T513'
set autotrace offResults (after set feedback off)
SQL> @temp
Execution Plan
Plan hash value: 1429545322
| Id  | Operation                   | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |       |    27 |   675 |    10   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T1    |    27 |   675 |    10   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | T1_I1 |    27 |       |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - access("TEST_USER"."PL_FUNC"("OBJECT_NAME")='T513')
SQL> spool offRegards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk