搞水位线的实验

-- 创建test3表
SQL> create table test3 as
  2  select * from dba_objects where 1 = 2;
Table created
-- 查看表中分配块，区大小
SQL> SELECT segment_name, segment_type, blocks -- 分配数据块数, extents -- 分配区块数
  2    FROM dba_segments
  3   WHERE segment_name = 'TEST3'
  4  ;
SEGMENT_NAME                                                                     SEGMENT_TYPE           BLOCKS    EXTENTS
-------------------------------------------------------------------------------- ------------------ ---------- ----------
TEST3                                                                            TABLE                       8          1
TEST3                                                                            TABLE                       8          1
-- 分析表TEST3表
SQL> ANALYZE TABLE TEST3 ESTIMATE STATISTICS;
Table analyzed
-- 查询TEST3表高水位线
SQL> SELECT blocks -- 高水位线(占用TEST3表数据块数), empty_blocks -- TEST3表空闲块数, num_rows
  2    FROM user_tables
  3   WHERE table_name = 'TEST3';
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
---------- ------------ ----------
         0            7          0
-- 因为未向TEST3表中插入任何数据，因此此表的高水位线为0，现向TEST3表中插入数据再观察
SQL> insert into test3
  2  select * from dba_objects;
50361 rows inserted
SQL> commit;
Commit complete
-- 重新分析表
SQL> ANALYZE TABLE TEST3 ESTIMATE STATISTICS;
Table analyzed
-- 再次查看表中分配块，区大小
SQL> SELECT segment_name, segment_type, blocks, extents
  2    FROM dba_segments
  3   WHERE segment_name = 'TEST3'
  4  ;
SEGMENT_NAME                                                                     SEGMENT_TYPE           BLOCKS    EXTENTS
-------------------------------------------------------------------------------- ------------------ ---------- ----------
TEST3                                                                            TABLE                       8          1
TEST3                                                                            TABLE                     768         21
此时看到BLOCKS数已增长到768, 也就是Oracle分配给TEST3表768个数据块，21个区
-- 再次查看TEST3表高水位线
SQL> SELECT blocks, empty_blocks, num_rows
  2    FROM user_tables
  3   WHERE table_name = 'TEST3';
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
---------- ------------ ----------
       689           78      50361
已增长到689个块， 还有78个空闲块，689 + 78 = 767, 比分配的少1个数据块，是因为这一个数据块是用作segment header
-- 现将TEST3表delete，在查看高水位线
SQL> delete from test3;
50361 rows deleted
SQL> commit;
Commit complete
SQL> ANALYZE TABLE TEST3 ESTIMATE STATISTICS;
Table analyzed
SQL>
SQL> SELECT blocks, empty_blocks, num_rows
  2    FROM user_tables
  3   WHERE table_name = 'TEST3';
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
---------- ------------ ----------
       689           78          0
发现此表高水位线并未减少，证明delete只是删除表中数据块的记录，但并不会使表中的高水位线下降， 在进行全表扫描时会Oracle会扫描表中高水位线下的所有数据块，
因此数据虽然被删除了，但查询时有可能还是很慢。所以在进行大表删除时应使用truncate语句，看下面实验:
SQL> truncate table test3;
Table truncated
SQL> ANALYZE TABLE TEST3 ESTIMATE STATISTICS;
Table analyzed
SQL>
SQL> SELECT blocks, empty_blocks, num_rows
  2    FROM user_tables
  3   WHERE table_name = 'TEST3';
    BLOCKS EMPTY_BLOCKS   NUM_ROWS
---------- ------------ ----------
         0            7          0