oracle延迟块清除

　　oracle在执行一些DML操作时，会在block上有活动事务的标志，如果一个事务commit后，由于某些block在commit之前已经写回datafile, 或者事务影响到的block数过多，则commit的时候只会清理undo segment header中的事务标志信息，data block上的事务标志不会清除，否则代价过高。那么在一些读取这些block时，需要将这些事务标志进行清除，就是延迟块清除

实验如下：

SQL> drop table t cascade constraints;

Table dropped.
--创建一张表t
SQL> create table t as select * from dba_objects where 1=2;

Table created.

SQL> select count(*) from t;

  COUNT(*)
----------
         0
--采用直接路径插入的方式，即不经过buffer cache
SQL> insert /*+ append */ into t select * from dba_objects;

87023 rows created.

SQL> set autotrace on;
SQL> commit; --提交，此时新插入数据的block上的事务标志并没有清除

Commit complete.

SQL> select count(*) from t;

  COUNT(*)
----------
     87023


Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522

-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |   339   (1)| 00:00:05 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T    | 67743 |   339   (1)| 00:00:05 |
-------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          0  recursive calls
          1  db block gets
       1249  consistent gets
       1241  physical reads
        168  redo size --查询即产生了redo，查询导致了data block上进行事务清除
        528  bytes sent via SQL*Net to client
        524  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> /

  COUNT(*)
----------
     87023


Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522

-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |   339   (1)| 00:00:05 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T    | 67743 |   339   (1)| 00:00:05 |
-------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       1246  consistent gets
          0  physical reads
          0  redo size  --再次查询不产生redo，事务已清除完毕
        528  bytes sent via SQL*Net to client
        524  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> truncate table t;

Table truncated.

SQL> select count(*) from t;

  COUNT(*)
----------
         0


Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522

-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T    |     1 |     2   (0)| 00:00:01 |
-------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          6  recursive calls
          1  db block gets
         13  consistent gets
          0  physical reads
         96  redo size --truncate表之后查询，同样也产生了延迟块清除
        525  bytes sent via SQL*Net to client
        524  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed