Oracle 中truncate与delete的区别

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truncate是oracle中独有的关键字吧!它的作用是清空一个表格,在删除数据方面,其与delete有一些区别,以便自己记住:

 


1、在功能上,truncate是清空一个表的内容,它相当于delete from table_name。
2、delete是dml操作,truncate是ddl操作;因此,用delete删除整个表的数据时,会产生大量的roolback,占用很多的rollback segments, 而truncate不会。
3、在内存中,用delete删除数据,表空间中其被删除数据的表占用的空间还在,便于以后的使用,另外它是“假相”的删除,相当于windows中用delete删除数据是把数据放到回收站中,还可以恢复,当然如果这个时候重新启动系统(OS或者RDBMS),它也就不能恢复了!
而用truncate清除数据,内存中表空间中其被删除数据的表占用的空间会被立即释放,相当于windows中用shift+delete删除数据,不能够恢复!
4、truncate 调整high water mark 而delete不;truncate之后,TABLE的HWM退回到 INITIAL和NEXT的位置(默认)delete 则不可以。
5、truncate 只能对TABLE,delete 可以是table,view,synonym。
6、TRUNCATE TABLE 的对象必须是本模式下的,或者有drop any table的权限 而 DELETE 则是对象必须是本模式下的,或被授予 DELETE ON SCHEMA.TABLE 或DELETE ANY TABLE的权限。
7、在外层中,truncate或者delete后,其占用的空间都将释放。
8、truncate和delete只删除数据,而drop则删除整个表(结构和数据)。 

 

delete 用法

Oracle Delete Statements 
Version 10.2 
  
Basic Delete Statements 
1.Delete All Rows:  DELETE <table_name>
                               or
                               DELETE FROM <table_name>; 
                               CREATE TABLE t  AS
                               SELECT *
                                FROM all_tables;

                                SELECT COUNT(*)
                                FROM t;

DELETE FROM t;

COMMIT;

SELECT COUNT(*)
FROM t;


2.Delete Selective Rows

 DELETE FROM <table_name>
WHERE <condition>; 
CREATE TABLE t AS
SELECT *
FROM all_tables;

SELECT COUNT(*)
FROM t;

DELETE FROM t
WHERE table_name LIKE ‘%MAP‘;

COMMIT;

SELECT COUNT(*)
FROM t;


3.Delete From A SELECT Statement

DELETE FROM (<SELECT Statement>); 
CREATE TABLE t AS
SELECT *
FROM all_tables;

SELECT COUNT(*)
FROM t;

DELETE FROM (
  SELECT * FROM t WHERE table_name LIKE ‘%MAP‘);

SELECT COUNT(*)
FROM t;


4.Delete With Returning Clause

 DELETE FROM (<SELECT Statement>); 
CREATE TABLE t AS
SELECT *
FROM all_tables;

set serveroutput on

DECLARE
 r  urowid; 
BEGIN
  DELETE FROM t
  WHERE rownum = 1
  RETURNING rowid INTO r;

  dbms_output.put_line(r);
END;

5.Delete Restricted To A Partition

DELETE FROM <table_name>
PARTITION <partition_name>; 
DELETE FROM sales PARTITION (q1_2001_invoices); 
Delete From A Remote Database DELETE FROM <table_name>@<database_link> 
DELETE FROM [email protected]_db
 

 在删除大数据量时(一个表中大部分数据时),

方法:
1、先将不需要删除的数据复制到一个临时表中
2、trunc table 表
3、将不需要删除的数据复制回来。

delete只循环了一次,(去查并删)
游标循环了二次,(先查出来,再循环一次删)
























































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