索引系列七--索引特性之高度较低是优化利器

sqlplus ljb/ljb

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drop table t1 purge;

drop table t2 purge;

drop table t3 purge;

drop table t4 purge;

drop table t5 purge;

drop table t6 purge;

drop table t7 purge;

create table t1 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1;

create table t2 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=10;

create table t3 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=100;

create table t4 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1000;

create table t5 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=10000;

create table t6 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=100000;

create table t7 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1000000;

create index idx_id_t1 on t1(id);

create index idx_id_t2 on t2(id);

create index idx_id_t3 on t3(id);

create index idx_id_t4 on t4(id);

create index idx_id_t5 on t5(id);

create index idx_id_t6 on t6(id);

create index idx_id_t7 on t7(id);

set linesize 1000

select index_name,

          blevel,

          leaf_blocks,

          num_rows,

          distinct_keys,

          clustering_factor

     from user_ind_statistics

    where table_name in( 'T1','T2','T3','T4','T5','T6','T7');

INDEX_NAME  BLEVEL LEAF_BLOCKS   NUM_ROWS DISTINCT_KEYS CLUSTERING_FACTOR

------------------ ----------- ---------- ------------- -----------------

IDX_ID_T1        0           1          1             1                 1

IDX_ID_T2        0           1         10            10                 2

IDX_ID_T3        0           1        100           100                15

IDX_ID_T4        1           3       1000          1000               143

IDX_ID_T5        1          21      10000         10000              1429

IDX_ID_T6        1         222     100000        100000             14286

IDX_ID_T7        2        2226    1000000       1000000            142858

set autotrace traceonly statistics

set linesize 1000

--以下注意观察逻辑读的次数，另外注意尽量每条语句执行2遍以上，观察第2遍的结果。

select * from t1 where id=1;  

统计信息

-----------------------------------------

          0  recursive calls

          0  db block gets

          2  consistent gets          

select /*+full(t1)*/ * from t1 where id=1; 

统计信息

-------------------------------

          0  recursive calls

          0  db block gets

          3  consistent gets 

select * from t2 where id=1; 

统计信息

-----------------------------

          0  recursive calls

          0  db block gets

          3  consistent gets           

select /*+full(t2)*/ * from t2 where id=1; 

统计信息

-----------------------------

          0  recursive calls

          0  db block gets

          5  consistent gets

select * from t3 where id=1; 

统计信息

-----------------------------

          0  recursive calls

          0  db block gets

          3  consistent gets

select /*+full(t3)*/ * from t3 where id=1; 

统计信息

----------------------------

          0  recursive calls

          0  db block gets

         19  consistent gets

select * from t4 where id=1; 

统计信息

-----------------------------

          0  recursive calls

          0  db block gets

          4  consistent gets

select /*+full(t4)*/ * from t4 where id=1; 

统计信息

----------------------------

          0  recursive calls

          0  db block gets

        148  consistent gets

select * from t5 where id=1;

统计信息

------------------------------

          0  recursive calls

          0  db block gets

          4  consistent gets

select /*+full(t5)*/ * from t5 where id=1; 

统计信息

-----------------------------

          0  recursive calls

          0  db block gets

       1435  consistent gets

select * from t6 where id=1;  

统计信息

-----------------------------

          0  recursive calls

          0  db block gets

          4  consistent gets  

select /*+full(t6)*/ * from t6 where id=1; 

统计信息

-----------------------------

          0  recursive calls

          0  db block gets

      14298  consistent gets

select * from t7 where id=1;  

统计信息

-----------------------------

          0  recursive calls

          0  db block gets

          5  consistent gets  

select /*+full(t7)*/ * from t7 where id=1; 

统计信息

-----------------------------

          0  recursive calls

          0  db block gets

     142866  consistent gets

/*

规律:

  从t1到t7(表记录依次增大10倍，从1到1000000),索引读的逻辑读是     2,3,3,4,4,4,5

  从t1到t7(表记录依次增大10倍，从1到1000000)全表扫描的逻辑读是    3,5,19,148,1435,14298,142866  

*/