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--MAX/MIN 的索引優(yōu)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;
執(zhí)行計劃
-------------------------------------------------------------------------------------------
| 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 |
-------------------------------------------------------------------------------------------
統計信息
----------------------------------------------------------
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
--最小值老師的試驗就無需展現執(zhí)行計劃結果了,必然和最大值的執(zhí)行計劃一樣!
select min(object_id) from t;
--如果沒用到索引的情況是如下,請看看執(zhí)行計劃有何不同,請看看代價和邏輯讀的差異!
select /*+full(t)*/ max(object_id) from t;
執(zhí)行計劃
---------------------------------------------------------------------------
| 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 |
---------------------------------------------------------------------------
統計信息
----------------------------------------------------------
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,可以做如下試驗觀察在有索引的情況下,隨這記錄數增加,性能差異是否明顯?
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;
執(zhí)行計劃
--------------------------------------------------------------------------------------------
| 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 |
--------------------------------------------------------------------------------------------
統計信息
----------------------------------------------------------
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如果允許為空,加個索引后,會走INDEX FULL SCAN (MIN/MAX)高效算法嗎,
當然會了!取最大最小還怕啥空值?
*/
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;
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