At 06:21 PM 10/3/2011, Angela liu wrote:
Thanks, what about if mysqld restart, does auto_increment gets reset ?
No. The next auto increment value stays with the table. As another
person already stated, you should never manually change the auto
increment value on a table that already has rows in it. MySQL always
handles the incrementing of an AutoInc field.
I saw this happened to Innodb, if table is empty and server restart,
auto_incremnet gets reset to 0
This shouldn't happen unless MySQL crashes. Perhaps you are
confusing the next Auto Increment value with the Last_Insert_Id()
(stored in server memory) which has a value only AFTER the user has
inserted a row. There can be a hundred connections (each with their
own session) adding rows to the same table at the same time.
Last_Insert_Id() will retrieve the autoinc value of the record that
was just inserted for YOUR session. You won't get someone else's
AutoInc value, only the autoinc value for the record that YOU just
inserted. So if you insert a record, wait 5 minutes, then execute a
"Select Last_Insert_Id()", you will get the correct autoinc value
that was used when YOUR record was inserted, even though another
hundred records were added while you waited to execute the Select
statement. You can never really reliably know what the autoinc value
will be for the record you are about to insert until AFTER the record
has been inserted and you execute Last_Insert_Id() to retrieve this
autoinc value.
I hope this clears it up.
Mike
From: mos <mo...@fastmail.fm>
To: mysql@lists.mysql.com
Sent: Monday, October 3, 2011 3:01 PM
Subject: Re: How MyISAM handle auto_increment
At 04:46 PM 10/3/2011, you wrote:
> Hi, Folks: I have questions regarding how MyISAM handles
auto_increment clolumn? 1. is there a auto_increment counter for
MyISAM to assign a new value to auto_increment columns?
Yes
> 2. if MyISAM has the counter, is the counter stored in memory or
disk? Thnaks
It is stored with the table definition. It is only reset to 0 when
the table is (re)created. You can get the last AutoInc for the
record that was just added by "Select Last_Insert_Id()". See
http://dev.mysql.com/doc/refman/5.0/en/example-auto-increment.html
Mike
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