Oracle dirty block的一些探讨(四)
继续探讨Oracle dirty话题，前面一直用到了alter system flush_cache命令。
那alter system checkpoint和alter system flush buffer_cache有什么区别？ 一个简单的测试可以看出些端倪。
打开SQLPUS 会话状态信息统计功能，可以看到对zhoul表格读取全部执行8个consistent gets，也就意味着表格全部在buffer cache中
SQL>  set autot traceonly stat
SQL>  select * from zhoul; 


Statistics
----------------------
          0  recursive calls
          0  db block gets
          8  consistent gets
          0  physical reads
          0  redo size
        517  bytes sent via SQL*Net to client
        400  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          3  rows processed
执行alter system checkpoint之后，可以看到表格读取依然执行8个consistent gets，也就意味着执行全量增量点后，dirty block在写数据文件的同时，内存并不清空。
SQL> alter system checkpoint;

System altered.

SQL> select * from zhoul;


Statistics
----------------------
          0  recursive calls
          0  db block gets
          8  consistent gets
          0  physical reads
          0  redo size
        517  bytes sent via SQL*Net to client
        400  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          3  rows processed
在执行flush buffer_cache之后，可以看到业务表格zhoul读取出现6个physical reads，也就意味着执行flush buffer_cache，Oracle会将buffer cache清空。
SQL> alter system flush buffer_cache;

System altered.

SQL>  select * from zhoul;


Statistics
----------------------
          0  recursive calls
          0  db block gets
          8  consistent gets
          6  physical reads
          0  redo size
        517  bytes sent via SQL*Net to client
        400  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          3  rows processed