select max(id),min(id) from table优化

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标题:select max(id),min(id) from table优化

作者:惜分飞©版权所有[未经本人同意,不得以任何形式转载,否则有进一步追究法律责任的权利.]

1、查看数据库版本
SQL> select * from v$version where rownum<2;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production

2、创建表和索引
create table t_a as select * from dba_objects;
create index t_a_ind on t_a(object_id);

3、查询最大值
SQL> select max(object_id) from t_a;
执行计划
———————————————————-
Plan hash value: 3226265922

————————————————————————————–

| 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)| T_A_IND | 1 | 13 | 2 (0)| 00:00:01 |

————————————————————————————–

Note—— dynamic sampling used for this statement (level=2)

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
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

4、查询最小值
SQL> select min(object_id) from t_a;
执行计划
———————————————————-
Plan hash value: 3226265922

————————————————————————————–

| 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)| T_A_IND | 1 | 13 | 2 (0)| 00:00:01 |

————————————————————————————–

Note—— dynamic sampling used for this statement (level=2)

统计信息
———————————————————-
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
429 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

5、查询最大值和最小值
SQL> select max(object_id),min(object_id) from t_a;
执行计划
———————————————————-
Plan hash value: 2127980459

—————————————————————————
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
—————————————————————————
| 0 | SELECT STATEMENT | | 1 | 13 | 293 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | TABLE ACCESS FULL| T_A | 78093 | 991K| 293 (1)| 00:00:04 |
—————————————————————————

Note—— dynamic sampling used for this statement (level=2)

统计信息
———————————————————-
4 recursive calls
0 db block gets
1119 consistent gets
1044 physical reads
0 redo size
502 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

6、查询最大值和最小值(排除null)
SQL> select max(object_id),min(object_id) from t_a where object_id is not null;
执行计划
———————————————————-
Plan hash value: 1214261695

———————————————————————————

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

———————————————————————————

| 0 | SELECT STATEMENT | | 1 | 13 | 50 (2)| 00:00:01
|

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

|* 2 | INDEX FAST FULL SCAN| T_A_IND | 78093 | 991K| 50 (2)| 00:00:01
|

———————————————————————————

Predicate Information (identified by operation id):
—————————————————

2 – filter(“OBJECT_ID” IS NOT NULL)

Note—— dynamic sampling used for this statement (level=2)

统计信息
———————————————————-
4 recursive calls
0 db block gets
242 consistent gets
0 physical reads
0 redo size
502 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

7、分别查询最大值和最小值
SQL> select (select max(object_id) from t_a) max,(select min(object_id) from t_a) min from dual;
执行计划
———————————————————-
Plan hash value: 312201770

————————————————————————————–

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

————————————————————————————–

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

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

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

| 3 | SORT AGGREGATE | | 1 | 13 | |

| 4 | INDEX FULL SCAN (MIN/MAX)| T_A_IND | 1 | 13 | 2 (0)| 00:00:01 |

| 5 | FAST DUAL | | 1 | | 2 (0)| 00:00:01 |

————————————————————————————–

Note—— dynamic sampling used for this statement (level=2)

统计信息
———————————————————-
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
480 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed

说明:
1、单查询最大值或者最小值,sql会自动走index
2、如果同时查询最大值和最小值,sql会使用全表扫描,而不是我们想象的索引快速扫描
3、加上where 排除掉null的情况,sql使用索引快速扫描,原因是:在不能确定索引列不为null(或者没有排除掉null)的情况下,不会使用索引快速扫描,而sql为了保证正确而采用了全表扫描
4、INDEX FULL SCAN (MIN/MAX)扫描效率很搞,所以把最大值,最小值分开查询,提高执行效率
5、其他写法
SQL> select (select /*+ index_asc(t_a t_a_ind) */ object_id from t_a where rownu
m=1) min ,(select /*+ index_desc(t_a t_a_ind) */ object_id from t_a where rownum=1)
max from dual;

执行计划
———————————————————-
Plan hash value: 674626822
—————————————————————————————

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

—————————————————————————————

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

|* 1 | COUNT STOPKEY | | | | | |

| 2 | INDEX FULL SCAN | T_A_IND | 78093 | 991K| 2 (0)| 00:

00:01 |

|* 3 | COUNT STOPKEY | | | | |

| 4 | INDEX FULL SCAN DESCENDING| T_A_IND | 78093 | 991K| 2 (0)| 00:

00:01 |

| 5 | FAST DUAL | | 1 | | 2 (0)| 00:00:01 |

—————————————————————————————

Predicate Information (identified by operation id):
—————————————————

1 – filter(ROWNUM=1)
3 – filter(ROWNUM=1)

Note—— dynamic sampling used for this statement (level=2)

统计信息
———————————————————-
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
480 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
注意:
1)这个要正确执行,需要一个前提条件object_id这列要为not null限制条件,不然会hint提示无效
2)从执行计划的统计信息上看,这个和INDEX FULL SCAN (MIN/MAX)方式的执行效率一样

此条目发表在 Oracle性能优化 分类目录。将固定链接加入收藏夹。

select max(id),min(id) from table优化》有 1 条评论

  1. 惜 分飞 说:

    hint中关于index相关的提示,需要确保index列不为空,或者有限制条件is not null,不然该hint提示不会生效

      [引用]  [回复]

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