Oracle的DML操作过程
用户将DML操作的语句通过进程传输给sga中的buffer cache，然后在buffer cache中对所更改的数据块进行更新操作，然后首先由logwr进
DML操作，Oracle所有进程配合执行的过程成！
用户将DML操作的语句通过进程传输给sga中的buffer cache，然后在buffer cache中对所更改的数据块进行更新操作，然后首先由logwr进程将此操作前的数据库传输给undo，将操作玩的数据传输给redo，此过程比较快（redo log为连续写）
然后再由dbwn进程将buffer cache中的脏数据块写入data file这个过程有间隔，这个间隔有ckpt进程来决定。
ckpt进程是如下运行的：
每隔3秒或更频繁写一次，写入控制文件和数据头文件，记录DBWN从SGA写入磁盘的块的位置（SCN（system change number） 系统更改号）
然后ckpt进程每次执行完之后，立刻通知dbwn进程，将现有的脏数据块写入data file
当dbwn进程将脏数据块写入data file后再产生一个检查点（checkpoint）
然后ckpt再次将scn更新到控制文件和data file的头文件中的scn之后，继续通知dbwn将buffer cache中的脏数据块写入到data file和dbwn创建检查点后继续等待ckpt进程的通知
如此循环，便是oracle的dml操作的过程！
后期补充：
另一位网友的介绍：
1、事务开始；
2、在buffer cache中找到需要的数据块，如果没有找到，则从数据文件中载入buffer cache中；
3、事务修改buffer cache的数据块，该数据被标识为“脏数据”，并被写入log buffer中；
4、事务提交，，LGWR进程将log buffer中的“脏数据”写入redo log file中；
5、当发生checkpoint，CKPT进程更新所有数据文件的文件头中的信息，DBWn进程则负责将Buffer Cache中的脏数据写入到数据文件中。
附:checkpoint由ckpt进程触发oracle进行checkpoint动作,将data buffer中的脏块(已经写在redo里记录但是没有写到datafile里的)的内容写入到data file里并释放站用的空间,由dbw后台进程完成,并修改controlfile和datafile的scn.
一般手工执行(alter system checkpoint)是由于要删除某个日志但是该日志里还有没有同步到data file里的内容,就需要手工check point来同步数据,然后就可以drop logfile group n.
相关阅读：
Oracle DML流程
PL/SQL“ ORA-14551: 无法在查询中执行 DML 操作”解决
MySQL常用DDL、DML、DCL语言整理（附样例）
Oracle基本事务和ForAll执行批量DML练习
Oracle DML语句(insert,update,delete) 回滚开销估算
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