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Oracle 全表扫描及其执行计划

全表扫描是Oracle访问数据库表是较为常见的访问方式之一。很多朋友一看到SQL语 句执行计划中的全表扫描，就要考虑对其进行修理一番。全表扫描的存在，的确存在可能优化的余地。但事实上很多时候全表扫描也并非是最低效的，完全要看不同 的情形与场合，任一方式都是有利有弊的，也就是具体情况要具体分析。本文描述了什么是全表扫描以及何时发生全表扫描，何时全表扫描才低效。

　　本文涉及到的相关链接：

　　高水位线和全表扫描

　　启用 AUTOTRACE 功能

　　Oracle 测试常用表BIG_TABLE

　　Oracle db_file_mulitblock_read_count参数

　　1、什么是全表扫描？

　　全表扫描就是扫表表中所有的行，实际上是扫描表中所有的数据块，因为Oracle中最小的存储单位是Oracle block。

　　扫描所有的数据块就包括高水位线以内的数据块，即使是空数据块在没有被释放的情形下也会被扫描而导致I/O增加。

　　在全表扫描期间，通常情况下，表上这些相邻的数据块被按顺序(sequentially)的方式访问以使得一次I/O可以读取多个数据块。

　　一次读取更多的数据块有助于全表扫描使用更少的I/O，对于可读取的数据块被限制于参数DB_FILE_MULTIBLOCK_READ_COUNT。

　　2、何时发生全表扫描？

　　a、表上的索引失效或无法被使用的情形(如对谓词使用函数、计算、NULL值、不等运算符、类型转换)

　　b、查询条件返回了整个表的大部分数据

　　c、使用了并行方式访问表

　　d、使用full 提示

　　e、统计信息缺失时使得Oracle认为全表扫描比索引扫描更高效

　　f、表上的数据块小于DB_FILE_MULTIBLOCK_READ_COUNT值的情形可能产生全表扫描

　　3、演示全表扫描的情形

a、准备演示环境
scott@ORA11G> select * from v$version where rownum<2;

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

--创建表t
scott@ORA11G> CREATE TABLE t
  2  AS
  3  SELECT rownum AS n, rpad('*',100,'*') AS pad
  4  FROM dual
  5  CONNECT BY level <= 1000;

Table created.

--添加索引
scott@ORA11G> create unique index t_pk on t(n);

Index created.

scott@ORA11G> alter table t add constraint t_pk primary key(n) using index t_pk;

Table altered.

--收集统计信息
scott@ORA11G> execute dbms_stats.gather_table_stats('SCOTT','T',cascade=>true);

PL/SQL procedure successfully completed.

scott@ORA11G> set autot trace exp;
scott@ORA11G> select count(*) from t;   --->count(*)的时候使用了索引快速扫描

Execution Plan
----------------------------------------------------------
Plan hash value: 454320086
----------------------------------------------------------------------
| Id  | Operation             | Name | Rows  | Cost (%CPU)| Time     |
----------------------------------------------------------------------
|   0 | SELECT STATEMENT      |      |     1 |     2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE       |      |     1 |            |          |
|   2 |   INDEX FAST FULL SCAN| T_PK |  1000 |     2   (0)| 00:00:01 |
----------------------------------------------------------------------

scott@ORA11G> set autot off;
scott@ORA11G> alter table t move;  --->进行move table

Table altered.

-->move 之后索引失效，如下所示
scott@ORA11G> @idx_info          
Enter value for owner: scott
Enter value for table_name: t

Table Name    INDEX_NAME     CL_NAM               CL_POS STATUS   IDX_TYP         DSCD
------------- -------------- -------------------- ------ -------- --------------- ----
T             T_PK           N                         1 UNUSABLE NORMAL          ASC


b、索引失效导致全表扫描
scott@ORA11G> set autot trace exp;
scott@ORA11G> select count(*) from t; 

Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522
-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     7   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T    |  1000 |     7   (0)| 00:00:01 |
-------------------------------------------------------------------

scott@ORA11G> set autot off;
scott@ORA11G> alter index t_pk rebuild;   -->重建索引

Index altered.

scott@ORA11G> @idx_info
Enter value for owner: scott
Enter value for table_name: t

Table Name     INDEX_NAME       CL_NAM               CL_POS STATUS   IDX_TYP         DSCD
-------------- ---------------- -------------------- ------ -------- --------------- ----
T              T_PK             N                         1 VALID    NORMAL          ASC


c、返回了整个表的大部分数据使用了全表扫描
scott@ORA11G> select count(pad) from t where n<=990;

Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |   105 |     7   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |      |     1 |   105 |            |          |
|*  2 |   TABLE ACCESS FULL| T    |   991 |   101K|     7   (0)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter("N"<=990)

--返回小部分数据时，使用的是索引扫描
scott@ORA11G> select count(pad) from t where n<=10;

Execution Plan
----------------------------------------------------------
Plan hash value: 4270555908
-------------------------------------------------------------------------------------
| Id  | Operation                    | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |      |     1 |   105 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE              |      |     1 |   105 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| T    |    10 |  1050 |     3   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | T_PK |    10 |       |     2   (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   3 - access("N"<=10)


d、使用并行方式访问表时使用了全表扫描
scott@ORA11G> select /*+ parallel(3) */ count(pad) from t where n<=10;

Execution Plan
----------------------------------------------------------
Plan hash value: 3126468333
----------------------------------------------------------------------------------------------------------------
| Id  | Operation              | Name     | Rows  | Bytes | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |          |     1 |   105 |     3   (0)| 00:00:01 |        |      |            |
|   1 |  SORT AGGREGATE        |          |     1 |   105 |            |          |        |      |            |
|   2 |   PX COORDINATOR       |          |       |       |            |          |        |      |            |
|   3 |    PX SEND QC (RANDOM) | :TQ10000 |     1 |   105 |            |          |  Q1,00 | P->S | QC (RAND)  |
|   4 |     SORT AGGREGATE     |          |     1 |   105 |            |          |  Q1,00 | PCWP |            |
|   5 |      PX BLOCK ITERATOR |          |    10 |  1050 |     3   (0)| 00:00:01 |  Q1,00 | PCWC |            |
|*  6 |       TABLE ACCESS FULL| T        |    10 |  1050 |     3   (0)| 00:00:01 |  Q1,00 | PCWP |            |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   6 - filter("N"<=10)
Note
-----
   - Degree of Parallelism is 3 because of hint
--Author : Robinson
--Blog   :http://blog.csdn.net/robinson_0612


e、使用full提示时使用了全表扫描
scott@ORA11G> select /*+ full(t) */ count(pad) from t where n<=10;

Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |   105 |     7   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |      |     1 |   105 |            |          |
|*  2 |   TABLE ACCESS FULL| T    |    10 |  1050 |     7   (0)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter("N"<=10)        


f、统计信息缺失导致全表扫描的情形
scott@ORA11G> exec dbms_stats.delete_table_stats('SCOTT','T');

PL/SQL procedure successfully completed.

scott@ORA11G> select count(pad) from t where n<=10;

Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |    65 |     7   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |      |     1 |    65 |            |          |
|*  2 |   TABLE ACCESS FULL| T    |    10 |   650 |     7   (0)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter("N"<=10)
Note
-----
   - dynamic sampling used for this statement (level=2)

--上面的执行计划使用了全表扫描，而且提示使用了动态采样，也就是缺乏统计信息
--表上的数据块小于DB_FILE_MULTIBLOCK_READ_COUNT值的情形可能产生全表扫描的情形不演示

　4、全表扫描何时低效？

--先来做几个实验
a、演示表上的相关信息
scott@ORA11G> @idx_info
Enter value for owner: scott
Enter value for table_name: big_table

Table Name                Index Name                CL_NAM    CL_POS Status   IDX_TYP         DSCD
------------------------- ------------------------- --------- ------ -------- --------------- ----
BIG_TABLE                 BIG_TABLE_PK              ID             1 VALID    NORMAL          ASC

scott@ORA11G> @idx_stat
Enter value for input_table_name: big_table
Enter value for owner: scott

                                                     AVG LEAF BLKS AVG DATA BLKS
BLEV IDX_NAME        LEAF_BLKS   DST_KEYS       PER KEY       PER KEY CLUST_FACT LAST_ANALYZED         TB_BLKS    TB_ROWS
---- -------------- ---------- ---------- ------------- ------------- ---------- ------------------ ---------- ----------
   1 BIG_TABLE_PK          208     100000             1             1       1483 20130524 10:45:51        1515     100000

--数据库参数设置
scott@ORA11G> show parameter optimizer_index_

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
optimizer_index_caching              integer     0
optimizer_index_cost_adj             integer     100
scott@ORA11G> show parameter optimizer_mode

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
optimizer_mode                       string      ALL_ROWS


b、查询返回20%数据行的情形
scott@ORA11G> alter system flush buffer_cache;                                                 
scott@ORA11G> select sum(object_id),avg(object_id) from big_table where id between 20000 and 40000;

Execution Plan
----------------------------------------------------------
Plan hash value: 3098837282                             -- 执行计划中，使用了索引范围扫描
---------------------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |     1 |    18 |   341   (0)| 00:00:05 |
|   1 |  SORT AGGREGATE              |              |     1 |    18 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| BIG_TABLE    | 20046 |   352K|   341   (0)| 00:00:05 |
|*  3 |    INDEX RANGE SCAN          | BIG_TABLE_PK | 20046 |       |    43   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   3 - access("ID">=20000 AND "ID"<=40000)
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
        351  consistent gets
        351  physical reads
          0  redo size
        427  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

scott@ORA11G> alter system flush buffer_cache;
scott@ORA11G> select /*+ full(big_table) */ sum(object_id),avg(object_id) from big_table where id between 20000 and 40000;

Execution Plan
----------------------------------------------------------
Plan hash value: 599409829                ---- 使用了提示执行为全表扫描
--------------------------------------------------------------------------------
| Id  | Operation          | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |           |     1 |    18 |   413   (1)| 00:00:05 |
|   1 |  SORT AGGREGATE    |           |     1 |    18 |            |          |
|*  2 |   TABLE ACCESS FULL| BIG_TABLE | 20046 |   352K|   413   (1)| 00:00:05 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter("ID"<=40000 AND "ID">=20000)
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       1486  consistent gets
       1484  physical reads
          0  redo size
        427  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

--注意对比上面两次操作中的consistent gets与physical reads


c、查询返回30%数据行的情形
scott@ORA11G> alter system flush buffer_cache;
scott@ORA11G> select sum(object_id),avg(object_id) from big_table where id between 20000 and 50000;

Execution Plan
----------------------------------------------------------
Plan hash value: 599409829             --->尽管返回数据的总行数为30%，而此时优化器使用了全表扫描
--------------------------------------------------------------------------------
| Id  | Operation          | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |           |     1 |    18 |   413   (1)| 00:00:05 |
|   1 |  SORT AGGREGATE    |           |     1 |    18 |            |          |
|*  2 |   TABLE ACCESS FULL| BIG_TABLE | 30012 |   527K|   413   (1)| 00:00:05 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter("ID"<=50000 AND "ID">=20000)
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       1486  consistent gets
       1484  physical reads
          0  redo size
        427  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

--下面使用提示来强制优化器走索引扫描
scott@ORA11G> alter system flush buffer_cache;
scott@ORA11G> select /*+ index(big_table big_table_pk) */ sum(object_id),avg(object_id)
  2  from big_table where id between 20000 and 50000;

Execution Plan
----------------------------------------------------------
Plan hash value: 3098837282
---------------------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |     1 |    18 |   511   (1)| 00:00:07 |
|   1 |  SORT AGGREGATE              |              |     1 |    18 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| BIG_TABLE    | 30012 |   527K|   511   (1)| 00:00:07 |
|*  3 |    INDEX RANGE SCAN          | BIG_TABLE_PK | 30012 |       |    64   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   3 - access("ID">=20000 AND "ID"<=50000)
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
        526  consistent gets
        526  physical reads
          0  redo size
        427  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
         
--注意观察每一次测试时所耗用的物理读与逻辑读
--从上面的测试可以看出，当表上所返回的数据行数接近于表上的30%时，Oracle 倾向于使用全表扫描
--而对于表上所返回的数据行数接近于表上的30%的情形，我们给与索引提示，此时比全表扫描更高效，即全表扫描是低效的
--笔者同时测试了数据返回总行数接近80%的情形以及创建了一个百万记录的进行对比测试
--大致结论，如果查询所返回的数据的总行数仅仅是表上数据的百分之八十以下，而使用了全表扫描，即可认为该全表扫描是低效的
--注：
--具体情况需要具体分析，如果你的表是千万级的，返回总数据的百分之零点几都会导致很大的差异
--其次，表上的索引应具有良好的聚簇因子，如不然，测试的结果可能有天壤之别
--最后，上面所描述的返回总行数应与执行结果返回的行数有差异，是指多少行参与了sum(object_id)

　5、小表的全表扫描是否高效？

--使用scott下dept表，仅有4行数据
scott@ORA11G> select * from dept where deptno>10;

3 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 2985873453            --->执行计划选择了索引扫描
---------------------------------------------------------------------------------------
| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |         |     3 |    60 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| DEPT    |     3 |    60 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | PK_DEPT |     3 |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("DEPTNO">10)
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          4  consistent gets                      -->使用了4次逻辑读
          0  physical reads
          0  redo size
        515  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          3  rows processed

-->下面强制使用全表扫描
scott@ORA11G> select /*+ full(dept) */ * from dept where deptno>10;

3 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 3383998547
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     3 |    60 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| DEPT |     3 |    60 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("DEPTNO">10)
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          4  consistent gets         -->此时的逻辑读同样为4次
          0  physical reads
          0  redo size
        515  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          3  rows processed

--下面来看看count(*)的情形
scott@ORA11G> select count(*) from dept;

1 row selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 3051237957               --->执行计划选择了索引全扫描
--------------------------------------------------------------------
| Id  | Operation        | Name    | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT |         |     1 |     1   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE  |         |     1 |            |          |
|   2 |   INDEX FULL SCAN| PK_DEPT |     4 |     1   (0)| 00:00:01 |
--------------------------------------------------------------------
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          1  consistent gets            -->逻辑读仅为1次
          0  physical reads
          0  redo size
        335  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

-->下面强制使用全表扫描
scott@ORA11G> select /*+ full(dept) */ count(*) from dept;

1 row selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 315352865
-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| DEPT |     4 |     3   (0)| 00:00:01 |
-------------------------------------------------------------------
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          3  consistent gets     -->使用了3次逻辑读
          0  physical reads
          0  redo size
        335  bytes sent via SQL*Net to client
        349  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
                   
--对于小表，从上面的情形可以看出，使用索引扫描也是比全表扫描高效
--因此，建议始终为小表建立索引

posted on 2013-06-07 10:10 顺其自然EVO 阅读(283) 评论(0)  编辑  收藏 所属分类: 数据库


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