Glitch in php or count() in mysql?

X-Followup: comp.lang.php

I have a PHP script that adds messages to a simple MySQL Database.
(PHP 5.0.3, MySQL 4.1.1)

One of the fields it stores is msgid.
The new msgid is a count of all current msgs in the db plus one

$query = 'select count(*) from messagesdb;';
$result = mysql_query ($query, $conn);
$msgid = mysql_result ($result, 'count(*)') + 1;

The next message is added using the above msgid.
For some reason (there are NO other scripts/systems accessing this
table, it is all on a local testmachine) I now have about 200 messages
in the system, but some id's occur more than once, up to 4 times.

I cannot explain this behaviour. I know I could avoid the entire issue
by autonumbering the messages, but still there is something funny going on.

I am wondering, is php messing up, or is this a mysql glitch, or am I
missing something here ?

Thanks for your time
Sh

Schraalhans Keukenmeester wrote:
> X-Followup: comp.lang.php
>
> I have a PHP script that adds messages to a simple MySQL Database.
> (PHP 5.0.3, MySQL 4.1.1)
>
> One of the fields it stores is msgid.
> The new msgid is a count of all current msgs in the db plus one
>
> $query = 'select count(*) from messagesdb;';
> $result = mysql_query ($query, $conn);
> $msgid = mysql_result ($result, 'count(*)') + 1;
>
> The next message is added using the above msgid.
> For some reason (there are NO other scripts/systems accessing this
> table, it is all on a local testmachine) I now have about 200 messages
> in the system, but some id's occur more than once, up to 4 times.
>
> I cannot explain this behaviour. I know I could avoid the entire issue
> by autonumbering the messages, but still there is something funny going on.
>
> I am wondering, is php messing up, or is this a mysql glitch, or am I
> missing something here ?

Race condition? If user 1 increments the $msgid and user 2 selects
count(*) before user 1 has added her message -> duplicate entries for
this $msgid. You can avoid this by putting a unique constraint on your
message ID. Which you should have done anyway ... Or using transactions.
Or autoincrement. Or you could, of course, re-invent other wheels.

Schraalhans Keukenmeester wrote:
> X-Followup: comp.lang.php
>
> I have a PHP script that adds messages to a simple MySQL Database.
> (PHP 5.0.3, MySQL 4.1.1)
>
> One of the fields it stores is msgid.
> The new msgid is a count of all current msgs in the db plus one
>
> $query = 'select count(*) from messagesdb;';
> $result = mysql_query ($query, $conn);
> $msgid = mysql_result ($result, 'count(*)') + 1;
>
> The next message is added using the above msgid.
> For some reason (there are NO other scripts/systems accessing this
> table, it is all on a local testmachine) I now have about 200 messages
> in the system, but some id's occur more than once, up to 4 times.


Why don't you make your msgid column in the mysql to an AUTO_INCREMENT?
There is a flaw in your code that can cause problems if there happens to be
more than one person who adds something to the database, the

$query = 'select count(*) from messagesdb;';

may be executed more than once before a new row is inserted, which leads to
multiple use of the same msgid.

When you insert your 201st row and then delete row 198, and then insert a new
row again, the id will be 201 again and you will end up with multiples of the
same msgid.

You should add the UNIQUE to the megid and maybe even PRIMARY KEY too, so that
you will not have this kind of troubles of multiple msgids.


> I am wondering, is php messing up, or is this a mysql glitch, or am I
> missing something here ?

it's the php code you write that most likely is the cause of the trouble.


//Aho

Schraalhans Keukenmeester wrote:
> X-Followup: comp.lang.php
>
> I have a PHP script that adds messages to a simple MySQL Database.
> (PHP 5.0.3, MySQL 4.1.1)
>
> One of the fields it stores is msgid.
> The new msgid is a count of all current msgs in the db plus one
>
> $query = 'select count(*) from messagesdb;';
> $result = mysql_query ($query, $conn);
> $msgid = mysql_result ($result, 'count(*)') + 1;
>
> The next message is added using the above msgid.
> For some reason (there are NO other scripts/systems accessing this
> table, it is all on a local testmachine) I now have about 200 messages
> in the system, but some id's occur more than once, up to 4 times.
>
> I cannot explain this behaviour. I know I could avoid the entire issue
> by autonumbering the messages, but still there is something funny going on.
>
> I am wondering, is php messing up, or is this a mysql glitch, or am I
> missing something here ?
>
> Thanks for your time
> Sh

In addition to what the others have said, if you ever delete a message
from the table, COUNT(*) will be decremented and you'll have duplicate
values.

--
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
==================

>I have a PHP script that adds messages to a simple MySQL Database.
>(PHP 5.0.3, MySQL 4.1.1)
>
>One of the fields it stores is msgid.
>The new msgid is a count of all current msgs in the db plus one

If you ever delete a message, this will mess up. It would work
better if you make the new msgid the maximum of the message ids in
the db plus one. This only screws up if you delete the previously
highest msgid.

There is also a possibility of messing up if two copies of the script
running at the same time both do their select, then both do their
inserts.

It's exactly this problem that auto_increment solves.

>$query = 'select count(*) from messagesdb;';
>$result = mysql_query ($query, $conn);
>$msgid = mysql_result ($result, 'count(*)') + 1;
>
>The next message is added using the above msgid.
>For some reason (there are NO other scripts/systems accessing this
>table, it is all on a local testmachine) I now have about 200 messages
>in the system, but some id's occur more than once, up to 4 times.
>
>I cannot explain this behaviour. I know I could avoid the entire issue
>by autonumbering the messages, but still there is something funny going on.
>
>I am wondering, is php messing up, or is this a mysql glitch, or am I
>missing something here ?

If the msgid is supposed to be unique, you should have a unique index
on it.

Gordon L. Burditt

"J.O. Aho" <user@example.net> wrote in message
news:3rusigFla8mtU1@individual.net...
> Schraalhans Keukenmeester wrote:
>> X-Followup: comp.lang.php
>>
>> I have a PHP script that adds messages to a simple MySQL Database.
>> (PHP 5.0.3, MySQL 4.1.1)
>>
>> One of the fields it stores is msgid.
>> The new msgid is a count of all current msgs in the db plus one
>>
>> $query = 'select count(*) from messagesdb;';
>> $result = mysql_query ($query, $conn);
>> $msgid = mysql_result ($result, 'count(*)') + 1;
>>
>> The next message is added using the above msgid.
>> For some reason (there are NO other scripts/systems accessing this
>> table, it is all on a local testmachine) I now have about 200 messages
>> in the system, but some id's occur more than once, up to 4 times.
>
>
> Why don't you make your msgid column in the mysql to an AUTO_INCREMENT?
> There is a flaw in your code that can cause problems if there happens to
> be
> more than one person who adds something to the database, the
>
> $query = 'select count(*) from messagesdb;';
>
> may be executed more than once before a new row is inserted, which leads
> to
> multiple use of the same msgid.
>
> When you insert your 201st row and then delete row 198, and then insert a
> new
> row again, the id will be 201 again and you will end up with multiples of
> the
> same msgid.

You should be using 'select max(msg_id) from messagesdb;' instead of 'select
count(*) from messagesdb'. In this way it will not matter if any previous
entries get deleted.

--
Tony Marston
http://www.tonymarston.net



> You should add the UNIQUE to the megid and maybe even PRIMARY KEY too, so
> that
> you will not have this kind of troubles of multiple msgids.
>
>
>> I am wondering, is php messing up, or is this a mysql glitch, or am I
>> missing something here ?
>
> it's the php code you write that most likely is the cause of the trouble.
>
>
> //Aho

Tony Marston wrote:

> You should be using 'select max(msg_id) from messagesdb;' instead of 'select
> count(*) from messagesdb'. In this way it will not matter if any previous
> entries get deleted.

This can still generate doublets of msgid as the select statment can be
trigged by two different people trying to add something,
AUTO_INCREMENT UNIQUE should in most cases be used for ID (numerial) columns.


//Aho

"J.O. Aho" <user@example.net> wrote in message
news:3s1l8jFln4j6U1@individual.net...
> Tony Marston wrote:
>
>> You should be using 'select max(msg_id) from messagesdb;' instead of
>> 'select
>> count(*) from messagesdb'. In this way it will not matter if any previous
>> entries get deleted.

Not if you lock the table bfeore performing the 'select max(id)' and the
'insert'.

--
Tony Marston

http://www.tonymarston.net



> This can still generate doublets of msgid as the select statment can be
> trigged by two different people trying to add something,
> AUTO_INCREMENT UNIQUE should in most cases be used for ID (numerial)
> columns.
>
>
> //Aho