oracle sql调优学习笔记（四）各种index access的方式

select /*+ index(employee IDX_EMP_1) */employee_id from employee;

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

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| 0 | SELECT STATEMENT | | 10000 | 40000 | 16 (7)| 00:00:01 |

| 1 | TABLE ACCESS FULL| EMPLOYEE | 10000 | 40000 | 16 (7)| 00:00:01 |

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计算成本，全盘扫描更合适

alter table employee modify(employee_id not null);

select employee_id from employee;

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

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| 0 | SELECT STATEMENT | | 10000 | 40000 | 6 (0)| 00:00:01 |

| 1 | INDEX FAST FULL SCAN| IDX_EMP_1 | 10000 | 40000 | 6 (0)| 00:00:01 |

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select /*+ index(employee IDX_EMP_1) */employee_id from employee;

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

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| 0 | SELECT STATEMENT | | 10000 | 40000 | 21 (5)| 00:00:01 |

| 1 | INDEX FULL SCAN | IDX_EMP_1 | 10000 | 40000 | 21 (5)| 00:00:01 |

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select /*+ index_ffs(employee IDX_EMP_1) */employee_id from employee;对指定的表执行快速全索引扫描,而不是全表扫描的办法.

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

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| 0 | SELECT STATEMENT | | 10000 | 40000 | 6 (0)| 00:00:01 |

| 1 | INDEX FAST FULL SCAN| IDX_EMP_1 | 10000 | 40000 | 6 (0)| 00:00:01 |

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drop index idx_emp_1;

create index idx_emp_2 on employee(gender,employee_id);

select * from employee where employee_id=101; SKIP SCAN是当索引的第一列不是where条件里的列,但索引的确包含了where条件里的列名,那么这时候就会用到skip
scan,字面意思理解就是跳过没用到的列,抵达where条件引用的列名上!

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

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| 0 | SELECT STATEMENT | | 1 | 5 | 3 (0)| 00:00:01 |

|* 1 | INDEX SKIP SCAN | IDX_EMP_2 | 1 | 5 | 3 (0)| 00:00:01 |

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select * from employee where employee_id=100;

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

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| 0 | SELECT STATEMENT | | | | 1 (100)| |

| 1 | TABLE ACCESS BY INDEX ROWID| EMPLOYEE | 1 | 15 | 1 (0)| 00:00:01 |

PLAN_TABLE_OUTPUT

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|* 2 | INDEX UNIQUE SCAN | IDX_UNI_EMP | 1 | | 0 (0)| |

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select * from employee where employee_id is null;

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

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| 0 | SELECT STATEMENT | | | | 15 (100)| |

|* 1 | TABLE ACCESS FULL| EMPLOYEE | 2 | 30 | 15 (0)| 00:00:01 |