索引系列十一--索引特性之有序与存列值优化max

--MAX/MIN 的索引优化

drop table t purge;

create table t as select * from dba_objects;

update t set object_id=rownum;

alter table t add constraint pk_object_id primary key (OBJECT_ID);

set autotrace on

set linesize 1000

select max(object_id) from t;

执行计划

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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 1 | 13 | 2 (0)| 00:00:01 |

| 1 | SORT AGGREGATE | | 1 | 13 | | |

| 2 | INDEX FULL SCAN (MIN/MAX)| PK_OBJECT_ID | 1 | 13 | 2 (0)| 00:00:01 |

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

统计信息

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

0 recursive calls

0 db block gets

2 consistent gets

0 physical reads

0 redo size

431 bytes sent via SQL*Net to client

415 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

--最小值老师的试验就无需展现执行计划结果了，必然和最大值的执行计划一样！

select min(object_id) from t;

--如果没用到索引的情况是如下,请看看执行计划有何不同，请看看代价和逻辑读的差异！

select /*+full(t)*/ max(object_id) from t;

执行计划

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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 1 | 13 | 292 (1)| 00:00:04 |

| 1 | SORT AGGREGATE | | 1 | 13 | | |

| 2 | TABLE ACCESS FULL| T | 92407 | 1173K| 292 (1)| 00:00:04 |

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

统计信息

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

0 recursive calls

0 db block gets

1047 consistent gets

0 physical reads

0 redo size

431 bytes sent via SQL*Net to client

415 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

---另外，可以做如下试验观察在有索引的情况下，随这记录数增加，性能差异是否明显？

set autotrace off

drop table t_max purge;

create table t_max as select * from dba_objects;

insert into t_max select * from t_max;

insert into t_max select * from t_max;

insert into t_max select * from t_max;

insert into t_max select * from t_max;

insert into t_max select * from t_max;

select count(*) from t_max;

create index idx_t_max_obj on t_max(object_id);

set autotrace on

select max(object_id) from t_max;

执行计划

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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 1 | 13 | 3 (0)| 00:00:01 |

| 1 | SORT AGGREGATE | | 1 | 13 | | |

| 2 | INDEX FULL SCAN (MIN/MAX)| IDX_T_MAX_OBJ | 1 | 13 | 3 (0)| 00:00:01 |

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

统计信息

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

0 recursive calls

0 db block gets

3 consistent gets

0 physical reads

0 redo size

431 bytes sent via SQL*Net to client

415 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

/*

object_id如果允许为空，加个索引后，会走INDEX FULL SCAN (MIN/MAX)高效算法吗，

当然会了！取最大最小还怕啥空值？

*/

drop table t purge;

create table t as select * from dba_objects ;

create index idx_object_id on t(object_id);

set autotrace on

set linesize 1000

select max(object_id) from t;