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执行计划中常见index访问方式

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近期有朋友对于单个表上的index各种情况比较模糊,这里对于单个表上,单个index出现的大多数情况进行了总结性测试,给出了测试结果,至于为什么出现这样的试验结果未做过多解释,给读者留下思考的空间.本篇文章仅仅是为了测试hint对index的影响,而不是说明走各种index方式的好坏.参考: INDEX FULL SCAN vs INDEX FAST FULL SCAN

创建表模拟测试

SQL> create table t_xifenfei as select object_id,object_name from dba_objects;

Table created.

SQL>  create index i_t_object_id on t_xifenfei(object_id); 

Index created.

SQL> exec dbms_stats.gather_table_stats(USER,\'T_XIFENFEI\',cascade=>true); 

PL/SQL procedure successfully completed.

SQL> desc t_xifenfei
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 OBJECT_ID                                          NUMBER
 OBJECT_NAME                                        VARCHAR2(128)

TABLE ACCESS FULL

SQL> SET AUTOT TRACE EXP STAT
SQL> SELECT OBJECT_ID FROM T_XIFENFEI;

49838 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 548923532

--------------------------------------------------------------------------------
| Id  | Operation         | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |            | 49838 |   243K|    57   (2)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| T_XIFENFEI | 49838 |   243K|    57   (2)| 00:00:01 |
--------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3544  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed

SQL> SELECT /*+ INDEX(T i_t_object_id) */ OBJECT_ID FROM T_XIFENFEI;

49838 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 548923532

--------------------------------------------------------------------------------
| Id  | Operation         | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |            | 49838 |   243K|    57   (2)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| T_XIFENFEI | 49838 |   243K|    57   (2)| 00:00:01 |
--------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3544  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed

从上面的执行计划中可知,此时走了全表扫描. 由于我们需要查询的列为object_id,因此理论上只需要读取索引就应该可以返回所有数据,而此时为什么是全表扫描呢？这是因为NULL值与索引的特性所决定的.即null值不会被存储到B树索引.因此应该为表 t_xifenfei 的列 object_id 添加 not null 约束.

INDEX FAST FULL SCAN

SQL> alter table t_xifenfei modify(object_id not null);   

Table altered.

SQL> SELECT  object_id from t_xifenfei;

49838 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 2036340805

--------------------------------------------------------------------------------------
| Id  | Operation            | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |               | 49838 |   243K|    27   (4)| 00:00:01 |
|   1 |  INDEX FAST FULL SCAN| I_T_OBJECT_ID | 49838 |   243K|    27   (4)| 00:00:01 |
--------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3432  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed

INDEX FAST FULL SCAN:当在高速缓存中没有找到所需的索引块时,则根据db_file_multiblock_read_count的值进行多块读操作.对于索引的分支结构只是简单的获取,然后扫描所有的叶结点.其结果是导致索引结构没有访问,获取的数据没有根据索引键的顺序排序.INDEX FAST FULL SCAN使用multiblock_read,故产生db file scattered reads 事件.

INDEX RANGE SCAN

SQL> select object_id from t_xifenfei where object_id<10;

8 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 2197008162

----------------------------------------------------------------------------------
| Id  | Operation        | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |               |     2 |    10 |     2   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| I_T_OBJECT_ID |     2 |    10 |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------------

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

   1 - access("OBJECT_ID"<10)

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        499  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          8  rows processed

SQL> select /*+ index_ffs(t i_t_object_id) */ object_id from t_xifenfei t where object_id<10;

8 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 2036340805

--------------------------------------------------------------------------------------
| Id  | Operation            | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |               |     2 |    10 |    27   (4)| 00:00:01 |
|*  1 |  INDEX FAST FULL SCAN| I_T_OBJECT_ID |     2 |    10 |    27   (4)| 00:00:01 |
--------------------------------------------------------------------------------------

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

   1 - filter("OBJECT_ID"<10)

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        118  consistent gets
          0  physical reads
          0  redo size
        499  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          8  rows processed
1
这里可以看出index_ffs已经生效,但是对于这样的情况hint index_ffs效率一般来说不会太高.


INDEX FULL SCAN
1
SQL> SELECT /*+ INDEX(T i_t_object_id) */ object_id  from t_xifenfei t;

49838 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 431110666

----------------------------------------------------------------------------------
| Id  | Operation        | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |               | 49838 |   243K|   113   (2)| 00:00:02 |
|   1 |  INDEX FULL SCAN | I_T_OBJECT_ID | 49838 |   243K|   113   (2)| 00:00:02 |
----------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3426  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed

INDEX FULL SCAN:完全按照索引存储的顺序依次访问整个索引树.当访问到叶结点之后,按照双向链表方式读取相连节点的值.换言之,对于索引上所有的数据是按照有序的方式来读取的.如果索引块没有在高速缓存中被找到时,则需要从数据文件中单块进行读取.对于需要读取大量数据的全索引扫描而言,这将使其变得低效.INDEX FULL SCAN使用single read,故产生db file sequential reads事件.新版的Oracle支持db file parallel reads方式.

HINT INDEX不会使用INDEX FAST FULL SCAN功能.

INDEX列ORDER BY

SQL> SELECT OBJECT_ID FROM T_XIFENFEI order by object_id ;

49838 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 431110666

----------------------------------------------------------------------------------
| Id  | Operation        | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |               | 49838 |   243K|   113   (2)| 00:00:02 |
|   1 |  INDEX FULL SCAN | I_T_OBJECT_ID | 49838 |   243K|   113   (2)| 00:00:02 |
----------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3426  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed

SQL> SELECT OBJECT_ID FROM T_XIFENFEI order by object_id  desc;

49838 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 2808014233

--------------------------------------------------------------------------------------------
| Id  | Operation                  | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |               | 49838 |   243K|   113   (2)| 00:00:02 |
|   1 |  INDEX FULL SCAN DESCENDING| I_T_OBJECT_ID | 49838 |   243K|   113   (2)| 00:00:02 |
--------------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3427  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed

SQL> SELECT  /*+ index_ffs(t i_t_object_id) */ object_id from t_xifenfei t order by object_id;

49838 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 2527678987

-----------------------------------------------------------------------------------------------
| Id  | Operation             | Name          | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |               | 49838 |   243K|       |   185   (4)| 00:00:03 |
|   1 |  SORT ORDER BY        |               | 49838 |   243K|  1192K|   185   (4)| 00:00:03 |
|   2 |   INDEX FAST FULL SCAN| I_T_OBJECT_ID | 49838 |   243K|       |    27   (4)| 00:00:01 |
-----------------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        117  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
      49838  rows processed

对于index 列排序,默认情况下会使用INDEX FULL SCAN/INDEX FULL SCAN DESCENDING而不选择使用INDEX FAST FULL SCAN,因为INDEX FAST FULL SCAN获得数据后,还需要做一次SORT ORDER BY操作

INDEX FAST FULL SCAN+SORT AGGREGATE

SQL> SELECT  count(object_id) FROM T_XIFENFEI;

Execution Plan
----------------------------------------------------------
Plan hash value: 3095383276

-------------------------------------------------------------------------------
| Id  | Operation             | Name          | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |               |     1 |    27   (4)| 00:00:01 |
|   1 |  SORT AGGREGATE       |               |     1 |            |          |
|   2 |   INDEX FAST FULL SCAN| I_T_OBJECT_ID | 49838 |    27   (4)| 00:00:01 |
-------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        117  consistent gets
          0  physical reads
          0  redo size
        421  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> SELECT  /*+ INDEX(T i_t_object_id) */ count(object_id) FROM T_XIFENFEI t;

Execution Plan
----------------------------------------------------------
Plan hash value: 3079973526

--------------------------------------------------------------------------
| Id  | Operation        | Name          | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT |               |     1 |   113   (2)| 00:00:02 |
|   1 |  SORT AGGREGATE  |               |     1 |            |          |
|   2 |   INDEX FULL SCAN| I_T_OBJECT_ID | 49838 |   113   (2)| 00:00:02 |
--------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        111  consistent gets
          0  physical reads
          0  redo size
        421  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

sort aggregate通常发生在使用一些聚合函数的时候,sum(),avg(),min(),max(),count()等等,实际上sort aggregate不做真正的sort,并不会用到排序空间,而是通过一个全局变量＋全表或全索引扫描来实现.这样的操作在默认情况下使用INDEX FAST FULL SCAN

INDEX FULL SCAN (MIN/MAX)

SQL> SELECT  max(object_id) FROM T_XIFENFEI;

Execution Plan
----------------------------------------------------------
Plan hash value: 2939893782

--------------------------------------------------------------------------------------------
| Id  | Operation                  | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |               |     1 |     5 |     2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE            |               |     1 |     5 |            |          |
|   2 |   INDEX FULL SCAN (MIN/MAX)| I_T_OBJECT_ID | 49838 |   243K|     2   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          2  consistent gets
          0  physical reads
          0  redo size
        419  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL>  SELECT /*+ index_ffs(t i_t_object_id) */ max(object_id) FROM T_XIFENFEI t;

Execution Plan
----------------------------------------------------------
Plan hash value: 2939893782

--------------------------------------------------------------------------------------------
| Id  | Operation                  | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |               |     1 |     5 |    27   (4)| 00:00:01 |
|   1 |  SORT AGGREGATE            |               |     1 |     5 |            |          |
|   2 |   INDEX FULL SCAN (MIN/MAX)| I_T_OBJECT_ID | 49838 |   243K|    27   (4)| 00:00:01 |
--------------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          2  consistent gets
          0  physical reads
          0  redo size
        419  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

对于这样的查询INDEX FULL SCAN (MIN/MAX)明显是最优,但是此处奇怪的是使用了index_ffs提示无效,如果有知道的朋友,麻烦告知原因.

建议继续学习：

Mysql执行计划中的Using filesort (阅读：2281)

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文章信息

作者：惜分飞来源：惜分飞
标签：执行计划
发布时间：2012-04-22 14:48:01

建议继续学习

Mysql执行计划中的Using filesort

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