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2015年5月20日19:14:43 - 排序

1. 查看排序区内存的大小以及设置

实际排序所用到的内存、磁盘的统计信息:

pga_aggregate_target:此参数用来指定所有session总计可以使用最大PGA内存 olap:50% oltp:20%

sqlSQL> show parameter pga_aggre

NAME                                 TYPE                   VALUE
------------------------------------ ---------------------- ------------------------------
pga_aggregate_target                 big integer            0
SQL> show parameter workarea_size_policy

workarea_size_policy:此参数用于开关PGA内存自动管理功能 auto:自动分配sort_area_size 属于workarea 如果需要经常排序就需要把这个值设置大点

sqlSQL> show parameter workarea_size_policy

NAME                                 TYPE                   VALUE
------------------------------------ ---------------------- ------------------------------
workarea_size_policy                 string                 AUTO

select name, value from v$sysstat where name like ‘sort%’; select * from v$pgastat;

sqlSQL> select name , value from v$sysstat where name like 'sort%';

NAME                                                                                                                          VALUE
-------------------------------------------------------------------------------------------------------------------------------- ----------
sorts (memory)                                                                                                                 4283
sorts (disk)                                                                                                                      0
sorts (rows)                                                                                                                  40823

2. 比较以下操作

select * from customers;

sqlSQL> set autotrace traceonly
SQL> select * from customers;

已选择55500行。

已用时间:  00: 00: 01.93

执行计划
----------------------------------------------------------
Plan hash value: 2008213504

-------------------------------------------------------------------------------
| Id  | Operation         | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |           | 55500 |  9810K|   406   (1)| 00:00:05 |
|   1 |  TABLE ACCESS FULL| CUSTOMERS | 55500 |  9810K|   406   (1)| 00:00:05 |
-------------------------------------------------------------------------------


统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       5057  consistent gets
       1455  physical reads
          0  redo size
   10855625  bytes sent via SQL*Net to client
      41109  bytes received via SQL*Net from client
       3701  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      55500  rows processed

select * from customers order by cust_last_name;

sqlSQL> select * from customers order by cust_last_name;

已选择55500行。

已用时间:  00: 00: 01.93

执行计划
----------------------------------------------------------
Plan hash value: 2792773903

----------------------------------------------------------------------------------------
| Id  | Operation          | Name      | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |           | 55500 |  9810K|       |  2612   (1)| 00:00:32 |
|   1 |  SORT ORDER BY     |           | 55500 |  9810K|    12M|  2612   (1)| 00:00:32 |
|   2 |   TABLE ACCESS FULL| CUSTOMERS | 55500 |  9810K|       |   406   (1)| 00:00:05 |
----------------------------------------------------------------------------------------


统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       1459  consistent gets
       1454  physical reads
          0  redo size
    6278979  bytes sent via SQL*Net to client
      41109  bytes received via SQL*Net from client
       3701  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
      55500  rows processed

可以看到使用order by用到了额外的12M内存

3. 在cust_last_name创建b*索引

没建索引:

sqlexplain plan for select  cust_last_name  from customers order by cust_last_name;
sqlSQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------
Plan hash value: 2792773903

----------------------------------------------------------------------------------------
| Id  | Operation          | Name      | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |           | 55500 |   433K|       |   610   (1)| 00:00:08 |
|   1 |  SORT ORDER BY     |           | 55500 |   433K|   880K|   610   (1)| 00:00:08 |
|   2 |   TABLE ACCESS FULL| CUSTOMERS | 55500 |   433K|       |   405   (1)| 00:00:05 |
----------------------------------------------------------------------------------------

建立索引:

sqlcreate index lastname_idx on customers(cust_last_name);
sqlexplain plan for select /*+ index(c lastname_idx) */ cust_last_name  from customers order by cust_last_name;

执行计划:

sqlSQL> explain plan for select /*+ index(c lastname_idx) */ cust_last_name  from customers order by cust_last_name;

已解释。

已用时间:  00: 00: 00.00
SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------
Plan hash value: 3470560620

---------------------------------------------------------------------------------
| Id  | Operation        | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |              | 55500 |   433K|   143   (1)| 00:00:02 |
|   1 |  INDEX FULL SCAN | LASTNAME_IDX | 55500 |   433K|   143   (1)| 00:00:02 |
---------------------------------------------------------------------------------

可以发现在一列建立索引后,对该列进行排序操作不需要再执行排序操作,
他会根据索引来进行查询(索引中本来就已经排好序)

结论

建立索引可以节省排序操作的时间。

4. 查询前十条记录

通常我们都会:

sqlselect cust_last_name from customers where rownum<=20 order by 1;
sqlSQL> select cust_last_name from customers where rownum<=20 order by 1;

CUST_LAST_NAME
--------------------------------------------------------------------------------
Everett
Everett
Everett
Everett
Everett
Everett
Everett
Everett
Everett
Ruddy
Ruddy

CUST_LAST_NAME
--------------------------------------------------------------------------------
Ruddy
Ruddy
Ruddy
Ruddy
Ruddy
Ruddy
Ruddy
Ruddy
Ruddy

已选择20行。

这样是错误的,因为在oracle中where永远都是先执行的也就先取20条再排序。

正确的做法是使用子查询:

sqlselect cust_last_name from (select * from customers order by cust_last_name) where rownum<10;
sqlSQL>select cust_last_name from (select * from customers order by cust_last_name) where rownum<10;
CUST_LAST_NAME
--------------------------------------------------------------------------------
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron

CUST_LAST_NAME
--------------------------------------------------------------------------------
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron

已选择20行。

没建索引:

sqlPLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------
Plan hash value: 1285511559

---------------------------------------------------------------------------------------------
| Id  | Operation               | Name      | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT        |           |     9 |   198 |       |  2612   (1)| 00:00:32 |
|*  1 |  COUNT STOPKEY          |           |       |       |       |            |          |
|   2 |   VIEW                  |           | 55500 |  1192K|       |  2612   (1)| 00:00:32 |
|*  3 |    SORT ORDER BY STOPKEY|           | 55500 |  9810K|    12M|  2612   (1)| 00:00:32 |
|   4 |     TABLE ACCESS FULL   | CUSTOMERS | 55500 |  9810K|       |   406   (1)| 00:00:05 |
---------------------------------------------------------------------------------------------

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------

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

   1 - filter(ROWNUM<10)
   3 - filter(ROWNUM<10)

已选择17行。

建立索引后:

sqlPLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
Plan hash value: 3026242074

----------------------------------------------------------------------------------
| Id  | Operation         | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |              |     9 |   198 |     4   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY    |              |       |       |            |          |
|   2 |   VIEW            |              |     9 |   198 |     4   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN| LASTNAME_IDX |     9 |       |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------------


PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(ROWNUM<10)

已选择15行。

建立索引后,子查询效率明显提高

同时select cust_last_name from customers where rownum<=10 order by cust_last_name;和上面的结果已经大不相同。

sqlSQL> select cust_last_name from customers where rownum<=10 order by cust_last_name;

CUST_LAST_NAME
--------------------------------------------------------------------------------
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron
Aaron

已选择10行。

执行计划:

sqlSQL> explain plan for
  2  select cust_last_name from customers where rownum<=10 order by cust_last_name;

已解释。

已用时间:  00: 00: 00.00
SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
Plan hash value: 2820001957

---------------------------------------------------------------------------------
| Id  | Operation        | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |              |    10 |    80 |     2   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY   |              |       |       |            |          |
|   2 |   INDEX FULL SCAN| LASTNAME_IDX | 55500 |   433K|     2   (0)| 00:00:01 |
---------------------------------------------------------------------------------

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
---------------------------------------------------

   1 - filter(ROWNUM<=10)

已选择14行。

竟然不是刚刚那样乱的。

结论

建立索引能够对提高子查询的性能,并且查询前十条记录不再需要使用子查询, 因为建立好索引后已经是排好序的,只要根据索引从表中拿出前十条记录即可。

5. 分组与索引

首先先建立一个表

sqlSQL> create table s as select * from sales;
表已创建。

s表与sales表的数据完全是一致的但s表没有sales表上的索引。

然后我们看一下执行计划:

sqlSQL> explain plan for
  2  select cust_id,avg(amount_sold) from s group by cust_id;

已解释。

已用时间:  00: 00: 00.23
SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------
Plan hash value: 1912481676

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |   785K|    19M|  1271   (4)| 00:00:16 |
|   1 |  HASH GROUP BY     |      |   785K|    19M|  1271   (4)| 00:00:16 |
|   2 |   TABLE ACCESS FULL| S    |   785K|    19M|  1236   (1)| 00:00:15 |
---------------------------------------------------------------------------

Note

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------
-----
   - dynamic sampling used for this statement (level=2)

已选择13行。

我们再看一下sales表(即建立了索引的表):

sqlSQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------
Plan hash value: 2820001957

---------------------------------------------------------------------------------
| Id  | Operation        | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |              |    10 |    80 |     2   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY   |              |       |       |            |          |
|   2 |   INDEX FULL SCAN| LASTNAME_IDX | 55500 |   433K|     2   (0)| 00:00:01 |
---------------------------------------------------------------------------------

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------
---------------------------------------------------

   1 - filter(ROWNUM<=10)

已选择14行。

结论

group by的一列上建立索引group by 性能更佳。

下面是关于order by 升序降序的执行计划可以看出建立索引升降序对排序不会影响性能
升序:

sqlSQL> select cust_last_name from customers where rownum <= 10 order by cust_last_name;
SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------
Plan hash value: 2820001957

---------------------------------------------------------------------------------
| Id  | Operation        | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |              |    10 |    80 |     2   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY   |              |       |       |            |          |
|   2 |   INDEX FULL SCAN| LASTNAME_IDX | 55500 |   433K|     2   (0)| 00:00:01 |
---------------------------------------------------------------------------------

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------
---------------------------------------------------

   1 - filter(ROWNUM<=10)

已选择14行。

降序:

sqlSQL> select cust_last_name from customers where rownum <= 10 order by cust_last_name;

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------
Plan hash value: 1596600344

--------------------------------------------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |              |     9 |    72 |     2   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY              |              |       |       |            |          |
|   2 |   INDEX FULL SCAN DESCENDING| LASTNAME_IDX | 55500 |   433K|     2   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------
---------------------------------------------------

   1 - filter(ROWNUM<10)

已选择14行。

Chao
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