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 t@remote_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)