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

巴特西

oracle延迟块清除

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