Oracle CBO评估like的数据量

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   对Like，有两种形式的写法是按照数据总量的5%评估。

SQL> drop table test purge;

     79747

SQL> select count(1) from test where object_name like ‘test%‘;
  COUNT(1)
----------
         0
SQL> select count(1) from test where object_name like ‘%test%‘;
  COUNT(1)
----------
        12
SQL> select count(1) from test where object_name like ‘%test‘;
  COUNT(1)
----------
         2

SQL> set autotrace trace exp

   1 - filter("OBJECT_NAME" LIKE ‘%test‘ AND "OBJECT_NAME" IS NOT NULL)

--如果是百分号写在后面，不能按照5%的

SQL> select * from test where object_name like ‘t%‘;
执行计划
----------------------------------------------------------
Plan hash value: 1357081020
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |  1079 |   105K|   224   (1)| 00:00:04 |
|*  1 |  TABLE ACCESS FULL| TEST |  1079 |   105K|   224   (1)| 00:00:04 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("OBJECT_NAME" LIKE ‘t%‘)

Oracle CBO评估like的数据量

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原文地址：http://blog.csdn.net/stevendbaguo/article/details/43796149