ORACLE INDEX重建

一：考虑重建索引的场合

1：表上频繁发生update,delete操作

2：表上发生了alter table ..move操作（move操作导致了rowid变化）

二：判断重建索引的标准

  索引重建是否有必要，一般看索引是否倾斜的严重，是否浪费了空间；

  那应该如何才可以判断索引是否倾斜的严重，是否浪费了空间，如下：

1，        对索引进行结构分析

Analyze index indexname validate structure;

2, 在执行步骤1的session中查询index_stats表，不要到别的session去查询

   select height,DEL_LF_ROWS/LF_ROWS from index_stats;

3, 在步骤2查询出来的height>=4或者DEL_LF_ROWS/LF_ROWS>0.2的场合，该索引考虑重建；

Example:

   SQL> select count(*) from test_index;

         COUNT(*)

----------

                   2072327

SQL> analyze index pk_t_test validate structure;

Index analyzed

        SQL> select height,DEL_LF_ROWS/LF_ROWS from index_stats;

                     HEIGHT
        DEL_LF_ROWS/LF_ROWS

---------- -------------------

         3                   0

SQL> delete from test_index where rownum<250000;

          249999 rows deleted

        SQL> select height,DEL_LF_ROWS/LF_ROWS from index_stats;

                    HEIGHT
                DEL_LF_ROWS/LF_ROWS

---------- -------------------

                         3                   0

        SQL> analyze index pk_t_test validate structure;

                Index
analyzed

SQL> select height,DEL_LF_ROWS/LF_ROWS fro
4000
m index_stats;

                    HEIGHT
                 DEL_LF_ROWS/LF_ROWS

---------- -------------------

                 3                           0.0777430939338362

三：重建索引的方式

    1：drop 原来的索引，然后再创建索引;

    2：alter index indexname rebuild (online);

    方式一：耗时间，无法在24*7环境中实现

    方式二：比较快，可以在24*7环境中实现

    建议使用方式二

四：alter index rebuid内部过程和注意点

    1：alter index rebuild 和alter index rebuild online的区别

(1)        扫描方式不同

Rebuild以index fast full scan（or table full scan）方式读取原索引中的数据来构建一个新的索引，有排序的操作; rebuild online 执行表扫描获取数据，有排序的操作;

Rebuild  方式 (index fast full scan  or  table full scan  取决于统计信息的cost)

Eg1:

SQL> explain plan for alter index idx_policy_id2 rebuild;

Explained

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

PLAN_TABLE_OUTPUT

---------------------------------------------------------------------

| Id  | Operation              |  Name           |
Rows  | Bytes | Cost  |

---------------------------------------------------------------------

|   0 | ALTER INDEX STATEMENT  |                 |   999K|  4882K|  3219
|

|   1 |  INDEX BUILD NON UNIQUE| IDX_POLICY_ID2  |       |       |       |

|   2 |   SORT CREATE INDEX     |                 |   999K|  4882K|       |

|   3 |    INDEX FAST FULL SCAN | IDX_POLICY_ID2  |   999K|  4882K|       |

---------------------------------------------------------------------

Eg2:

SQL>  explain plan for alter index idx_policy_id rebuild;

Explained

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

PLAN_TABLE_OUTPUT

---------------------------------------------------------------------

| Id  | Operation              |  Name          |
Rows  | Bytes | Cost  |

---------------------------------------------------------------------

|   0 | ALTER INDEX STATEMENT  |                |  2072K|     9M|   461
|

|   1 |  INDEX BUILD NON UNIQUE| IDX_POLICY_ID  |       |       |       |

|   2 |   SORT CREATE INDEX    |                |  2072K|     9M|       |

|   3 |    TABLE ACCESS FULL   | TEST_INDEX     |  2072K|     9M|   461
|

Eg3： (注意和Eg1比较)

Rebuil online 方式：

SQL> explain plan for alter index idx_policy_id2 rebuild online;

Explained

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

PLAN_TABLE_OUTPUT

---------------------------------------------------------------------

| Id  | Operation              |  Name