删除并截断和 table 大小

Question

我尝试了此查询以了解删除和截断后对 table 大小的影响。

select * from SALES_HISTORY;
Output:
Product     Month       Sales
sony    22-DEC-17   24000
sony    22-DEC-17   24000
sony    22-DEC-17   24000
sony    22-DEC-17   24000
sony    22-DEC-17   24000
sony    22-DEC-17   24000

Delete from sales_history:
commit;

select segment_name,segment_type,bytes/1024/1024 MB
from dba_segments
where segment_type='TABLE' and segment_name='SALES_HISTORY';

Output:
segment_name    segment_type   MB
SALES_HISTORY   TABLE         0.0625

Truncate table SALES_HISTORY;

之后，我再次尝试查找 table 的大小，但似乎与下图相同。如果截断了所有行，它不应该为零吗？

Output:
segment_name    segment_type   MB
SALES_HISTORY   TABLE         0.0625

我需要了解删除和截断后对table和水印大小的影响。

Answer 1

如果您使用的是 11g R2 或更高版本，请查看 DROP ALL STORAGE 是否符合您的要求。看看下面的例子：

SQL> create table test_truncate as select level id, sysdate + level datum
  2  from dual connect by level <= 100000;

Table created.

SQL> select segment_name,segment_type,bytes/1024/1024 MB
  2  from dba_segments
  3  where segment_type='TABLE' and segment_name='TEST_TRUNCATE';

SEGMENT_NAME         SEGMENT_TYPE               MB
-------------------- ------------------ ----------
TEST_TRUNCATE        TABLE                       3

SQL> truncate table test_truncate;

Table truncated.

SQL> select segment_name,segment_type,bytes/1024/1024 MB
  2  from dba_segments
  3  where segment_type='TABLE' and segment_name='TEST_TRUNCATE';

SEGMENT_NAME         SEGMENT_TYPE               MB
-------------------- ------------------ ----------
TEST_TRUNCATE        TABLE                   ,0625

SQL> truncate table test_truncate DROP ALL STORAGE;

Table truncated.

SQL> select segment_name,segment_type,bytes/1024/1024 MB
  2  from dba_segments
  3  where segment_type='TABLE' and segment_name='TEST_TRUNCATE';

no rows selected

SQL>

Answer 2

truncate 有几个选项，下面是这些选项和 delete 如何影响存储和高水位线：

delete 将所有 space 分配给 table。高水位线保持不变
truncate table ... reuse storage 将所有 space 分配给 table。这会将高水位线重置为第一个范围
truncate table ... drop storage（默认值）为 table 释放 minextents 上方的所有 space。并因此重置高水位线
truncate table ... drop all storage 从 table

您可以使用 dbms_space.unused_space 来验证这一点：

create table t as 
  select level id, lpad ( 'x', 1000, 'x' ) stuff 
  from   dual
  connect by level <= 1000;

create or replace procedure show_size as

   out_total_blocks               integer;
   out_total_bytes                integer;
   out_unused_blocks              integer;
   out_unused_bytes               integer;
   out_last_used_extent_file_id   integer;
   out_last_used_extent_block_id  integer;
   out_last_used_block            integer;

begin

  dbms_space.unused_space(
    segment_owner              =>  user
    ,segment_name              =>  'T'
    ,segment_type              =>  'TABLE'
    ,total_blocks              =>  out_total_blocks
    ,total_bytes               =>  out_total_bytes 
    ,unused_blocks             =>  out_unused_blocks
    ,unused_bytes              =>  out_unused_bytes
    ,last_used_extent_file_id  =>  out_last_used_extent_file_id
    ,last_used_extent_block_id =>  out_last_used_extent_block_id
    ,last_used_block           =>  out_last_used_block
  );

  dbms_output.put_line ( 'Total Blocks:    ' ||  out_total_blocks || ' (blocks in the segment)');
  dbms_output.put_line ( 'Total Bytes:     ' ||  out_total_bytes || ' (segment size in bytes)');
  dbms_output.put_line ( 'Unused Blocks:   ' ||  out_unused_blocks || ' (empty blocks)');
  dbms_output.put_line ( 'Unused Bytes:    ' ||  out_unused_bytes || ' (size of empty space in bytes)');
  dbms_output.put_line ( 'Last Used Block: ' ||  out_last_used_block || ' (high water mark)');
end;
/ 

exec show_size;

Total Blocks:    256 (blocks in the segment)
Total Bytes:     2097152 (segment size in bytes)
Unused Blocks:   101 (empty blocks)
Unused Bytes:    827392 (size of empty space in bytes)
Last Used Block: 27 (high water mark)

delete t;
commit;

exec show_size;

Total Blocks:    256 (blocks in the segment)
Total Bytes:     2097152 (segment size in bytes)
Unused Blocks:   101 (empty blocks)
Unused Bytes:    827392 (size of empty space in bytes)
Last Used Block: 27 (high water mark)

truncate table t
  reuse storage;

exec show_size;

Total Blocks:    256 (blocks in the segment)
Total Bytes:     2097152 (segment size in bytes)
Unused Blocks:   253 (empty blocks)
Unused Bytes:    2072576 (size of empty space in bytes)
Last Used Block: 3 (high water mark)

truncate table t
  drop storage;

exec show_size;

Total Blocks:    8 (blocks in the segment)
Total Bytes:     65536 (segment size in bytes)
Unused Blocks:   5 (empty blocks)
Unused Bytes:    40960 (size of empty space in bytes)
Last Used Block: 3 (high water mark)

truncate table t
  drop all storage;

exec show_size;

Total Blocks:    0 (blocks in the segment)
Total Bytes:     0 (segment size in bytes)
Unused Blocks:   0 (empty blocks)
Unused Bytes:    0 (size of empty space in bytes)
Last Used Block: 0 (high water mark)

Answer 3

我想知道你没有包括 table 结构来计算大小。显示的大小是 table 的结构大小。

 Tables whole size= Tables structure 
 (Table name and fields (0.0625 mb)) + 
 (records/the 
  data (remaining size in mb)) as per the 
above data u showed

验证更改 table 并删除一个 column/field

  ALTER TABLE SALES_HISTORY DROP 
  Sales;

 select  
 segment_name,segment_type,
bytes/1024/1024 MB
from dba_segments
where segment_type='TABLE' and 
segment_name='SALES_HISTORY';

删除并截断和 table 大小

Delete and Truncate and table size

sql

oracle

truncate

sql-delete