--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;
