Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

CHAPTER 11 ■ INDEXES 431
The data in the index is not sorted by X before it is stored, but rather by REVERSE(X), hence
the range scan for X > 5 will not be able to use the index. On the other hand, some range scans
can be done on a reverse key index. If I have a concatenated index on (X, Y), the following
predicate will be able to make use of the reverse key index and will “range scan” it:
where x = 5
This is because the bytes for X are reversed, and then the bytes for Y are reversed. Oracle
does not reverse the bytes of (X || Y), but rather stores (REVERSE(X) || REVERSE(Y)). This
means all of the values for X=5will be stored together, so Oracle can range scan that index to
find them all.
Now, assuming you have a surrogate primary key on a table populated via a sequence,
and you do not need to use range scanning on this index—that is, you don’t need to query for
MAX(primary_key), MIN(primary_key), WHERE primary_key < 100, and so on—then you could
consider a reverse key index in high insert scenarios even in a single instance of Oracle. I set
up two different tests, one in a pure PL/SQL environment and one using Pro*C to demonstrate
the differences between inserting into a table with a reverse key index on the primary key and
one with a conventional index. In both cases, the table used was created with the following
DDL (we will avoid contention on table blocks by using ASSM so we can isolate the contention
on the index blocks):
create table t tablespace assm
as
select 0 id, a.*
from all_objects a
where 1=0;
alter table t
add constraint t_pk
primary key (id)
using index (create index t_pk on t(id) &indexType tablespace assm);
create sequence s cache 1000;
whereby &indexType was replaced with either the keyword REVERSE, creating a reverse key
index, or with nothing, thus using a “regular” index. The PL/SQL that would be run by 1, 2, 5,
10, or 15 users concurrently was
create or replace procedure do_sql
as
begin
for x in ( select rownum r, all_objects.* from all_objects )
loop
insert into t
( id, OWNER, OBJECT_NAME, SUBOBJECT_NAME,
OBJECT_ID, DATA_OBJECT_ID, OBJECT_TYPE, CREATED,
LAST_DDL_TIME, TIMESTAMP, STATUS, TEMPORARY,
GENERATED, SECONDARY )
values