innodb 中的并发
我有一个这样的代码
Reserve.php
$r=mysql_query("select count(*) from ticket");
$rec=mysql_fetch_array($r);
if ($rec[0]==0)
{
insert into ticket values .....
}
我只有 1 张票。 两个用户请求reserve.php。
“a”用户请求 Reserve.php 且可用票证为 0 。但在插入之前，对于“b”用户，可用票证还为 0。所以两个用户预订票。
表是Innodb。 如何防止这种情况？事务、锁表还是什么？
i have a code like this
reserve.php
$r=mysql_query("select count(*) from ticket");
$rec=mysql_fetch_array($r);
if ($rec[0]==0)
{
insert into ticket values .....
}
i have 1 ticket only.
two users request reserve.php.
"a" user request reserve.php and available ticket is 0 . but before insert, for "b" user available ticket is 0 yet. so two users reserve ticket.
table is Innodb.
how to prevent this? transaction , lock table or what?
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在这些情况下，我通常只使用 UPDATE 语句并检查有多少记录受到更新的影响 (mysql_affected_rows)。
如果其他人首先递减计数器，则不会发生更新，并且您不会向用户提供票证。应启用自动提交，以便更新是一个独立的“事务”。
或者，您可以将 SELECT 更改为 SELECT ... FOR UPDATE
http://dev.mysql.com/doc/refman /5.0/en/innodb-locking-reads.html
In these situations I usually just use an UPDATE statement and check how many records were affected (mysql_affected_rows) by the update.
If someone else decremented the counter first, then no update occurs and you don't give the user a ticket. Autocommit should be enabled so the update is a self-contained "transaction".
Alternatively, you can change your SELECT to be SELECT ... FOR UPDATE
http://dev.mysql.com/doc/refman/5.0/en/innodb-locking-reads.html
我会在单个事务中完成此操作，无需往返应用程序级别 -
SELECT count() ....和INSERT INTO均在单个命令中发送来自 PHP 并嵌入到带有EXCLUSIVE LOCK的TRANSACTION中I would do it in a single transaction, without a roundtrip back to the application level - both
SELECT count() ....andINSERT INTObeing sent in a single command from PHP and embedded into aTRANSACTIONwithEXCLUSIVE LOCK