如何降低索引的clustering_factor (转)

环境:
OS:Red Hat Linux As 5
DB:10.2.0.4
 
我们知道判断一个索引的好坏可以通过该索引的clustering_factor高低来衡量,clustering_factor越低,索引的使用效果就越好,那怎么样才能降低索引的clustering_factor呢,通常使用的方法是让表的索引字段值按顺序存储,下面通过一个例子说明.
 
1.创建表并构造无序的数据
create table scott.tb_index_test
(
  id number not null,
  name varchar2(30)
);
 
create index scott.idx_tb_index_test  on scott.tb_index_test(id);
 
declare
  l_random_value number;
begin
  for i in  1 .. 100000 loop
    l_random_value := round(dbms_random.value(1,100000));
    insert into scott.tb_index_test(id) values (l_random_value);
    commit;
  end loop;
end;
 
2.分析表
 
begin
  dbms_stats.gather_table_stats(ownname => 'SCOTT',
                                tabname => 'TB_INDEX_TEST',
                                cascade => true);
end;
 
3.查看索引当前的clustering_factor
 
SQL> select ui.clustering_factor, ui.num_rows, ui.index_type, ui.distinct_keys
  from dba_indexes ui where ui.table_name = 'TB_INDEX_TEST';
CLUSTERING_FACTOR   NUM_ROWS INDEX_TYPE                  DISTINCT_KEYS
----------------- ---------- --------------------------- -------------
            99742     100000 NORMAL                             100000
 
4.将表中的数据按照索引字段存储
 
create table scott.tmp as select * from scott.tb_index_test;
truncate table scott.tb_index_test;
insert into scott.tb_index_test select * from scott.tmp order by id;
 
5.再次分析表
 
begin
  dbms_stats.gather_table_stats(ownname => 'SCOTT',
                                tabname => 'TB_INDEX_TEST',
                                cascade => true);
end;
 
6.这个时候索引的clustering_factor明显降低了
 
SQL> select ui.clustering_factor, ui.num_rows, ui.index_type, ui.distinct_keys
  from dba_indexes ui where ui.table_name = 'TB_INDEX_TEST';
CLUSTERING_FACTOR   NUM_ROWS INDEX_TYPE                  DISTINCT_KEYS
----------------- ---------- --------------------------- -------------
              372     100000 NORMAL                             100000
 
posted @ 2013-04-15 21:47  wilson.han  阅读(247)  评论(0编辑  收藏  举报