MySQL的优化（七）

三十一、事务的例子

MyIASM表如何进行事务处理:

mysql> LOCK TABLES trans READ, customer WRITE;

mysql> select sum(value) from trans where customer_id=some_id;

mysql> update customer set total_value=sum_from_previous_statement

where customer_id=some_id;

mysql> UNLOCK TABLES;

BDB表如何进行事务:

mysql> BEGIN WORK;

mysql> select sum(value) from trans where customer_id=some_id;

mysql> update customer set total_value=sum_from_previous_statement

where customer_id=some_id;

mysql> COMMIT;

注意你可以通过下列语句回避事务:

UPDATE customer SET value=value+new_value WHERE customer_id=some_id;

三十二、使用REPLACE的例子

REPLACE的功能极像INSERT，除了如果一条老记录在一个唯一索引上具有与新纪录相同的值，那么老记 录在新纪录插入前则被删除。不使用 SELECT 1 FROM t1 WHERE key=#

IF found-row

LOCK TABLES t1

DELETE FROM t1 WHERE key1=#

INSERT INTO t1 VALUES (...)

UNLOCK TABLES t1;

ENDIF

而用

REPLACE INTO t1 VALUES (...)