Oracle降低高水位

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       1.创建测试表
       
       
SQL> create table t1 (id int);
       
       

       
       
Table created.
       
       

       
       

       
       
2.查看表占用的块数量
       
       
SQL> SELECT blocks, empty_blocks, num_rows FROM dba_tables WHERE table_name='T1';
       
       

       
       
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
       
       
---------- ------------ ----------
       
       

       
       
3.对T1表做统计信息收集
       
       
SQL> ANALYZE TABLE T1 COMPUTE STATISTICS;
       
       

       
       
Table analyzed.
       
       

       
       

       
       
4.再次查看表占用的块数量
       
       
SQL> SELECT blocks, empty_blocks, num_rows FROM dba_tables WHERE table_name='T1';
       
       

       
       
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
       
       
---------- ------------ ----------
       
       
     0          7      0
       
       

       
       
5.向表中插入1000W条数据
       
       
declare
       
       
i number;
       
       
begin
       
       
for i in 1..10000000 loop
       
       
 insert into t1 values(i);
       
       
end loop;
       
       
commit;
       
       
end;
       
       
/
       
       

       
       

       
       
6.查看表占用的块数量
       
       
SQL> SELECT blocks, empty_blocks, num_rows FROM dba_tables WHERE table_name='T1';
       
       

       
       
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
       
       
---------- ------------ ----------
       
       
     0          7      0
       
       

       
       
7.统计信息收集
       
       
SQL> ANALYZE TABLE T1 COMPUTE STATISTICS;
       
       

       
       
Table analyzed.
       
       

       
       
8.再次查看表占用的块数量
       
       
SQL> SELECT blocks, empty_blocks, num_rows FROM dba_tables WHERE table_name='T1';
       
       

       
       
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
       
       
---------- ------------ ----------
       
       
     15155        204   10000000
       
       

       
       
9.删除数据
       
       
SQL> delete from t1;
       
       

       
       
10000000 rows deleted.
       
       

       
       
SQL> commit;
       
       

       
       
Commit complete.
       
       

       
       

       
       
10.查看表占用的块数量
       
       
SQL> SELECT blocks, empty_blocks, num_rows FROM dba_tables WHERE table_name='T1';
       
       

       
       
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
       
       
---------- ------------ ----------
       
       
     15155        204   10000000
       
       

       
       
11.统计信息收集
       
       
SQL> ANALYZE TABLE T1 COMPUTE STATISTICS;
       
       

       
       
Table analyzed.
       
       

       
       

       
       
12.再次查看表占用的块数量，可以看到
       
       
SQL> SELECT blocks, empty_blocks, num_rows FROM dba_tables WHERE table_name='T1';
       
       

       
       
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
       
       
---------- ------------ ----------
       
       
     15155        204      0
       
       

       
       
13.降低高水位操作
       
       
SQL> alter table T1 move;
       
       

       
       
Table altered.
       
       

       
       

       
       
14.再次查看表占用的块数量
       
       
SQL> SELECT blocks, empty_blocks, num_rows FROM dba_tables WHERE table_name='T1';
       
       

       
       
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
       
       
---------- ------------ ----------
       
       
     15155        204      0
       
       

       
       

       
       
15.再次收集统计信息
       
       
SQL> ANALYZE TABLE T1 COMPUTE STATISTICS;
       
       

       
       
Table analyzed.
       
       

       
       
16.查看表占用的块数量，可以看到执行alter table T1 move命令后，高水位已经下降了。
       
       
SQL> SELECT blocks, empty_blocks, num_rows FROM dba_tables WHERE table_name='T1';
       
       

       
       
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
       
       
---------- ------------ ----------
       
       
     0          7      0
      
      
      
      

       
       
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4种降低高水位的方法：

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       ANALYZE TABLE T1 COMPUTE STATISTICS;      分析表
       
       
SELECT blocks, empty_blocks, num_rows FROM dba_tables WHERE table_name='T1';   查询高水位线方法
       
       
 1：alter table T1 enable row movement;    开启行迁移(收缩表操做第一步)
       
       
 alter table T1 shrink space ;     收缩表空间
       
       
 alter table T1 disable row movement;   关闭行迁移方法 
       
       
 2：alter table T1 move;    移动表需要新建索引
       
       
 create Index new_index On T1(deptno);   单一索引
       
       
 create Index new_index  on T1(deptno,job);   复合索引方法 
       
       
 3：truncate table T1;  清空表方法 
       
       
 4：create table T1_NEW as select * from T1 where 1=2;
       
       
 创建新表alter table T1_NEW rename to T1;    重命名新表
      
      
      
      

       
       
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