Mysql – Insert the same time, it is possible to happen

bulk-insertinsertMySQLPHP

Good evening.

I have a doubt about inclusion in the database. A common example is a user account where the user names must be unique.

In the following logic:

User enters data into a form.
Data are collected via $ _POST (or other method)
Before saving the data is checked if the user name is already registered
If not, save the record, if it is, it informs the user that the username that already exists.

Example:

<?php
$username = $_POST['username'];
$query = "SELECT * FROM `user_tbl` WHERE `username` = '{$username}'";
$result = mysql_query($query);
if ( mysql_num_rows ( $result ) > 1 ) {
    /* Username already exists */
    echo 'Username already exists';
} else {
    /* Username doesn't exist */
    /* a certain time is elapsed after checking */
    /* .. insert query */
}

My question is, in a system with high volume of requests, it is possible that between the time to check whether the user name already exists and the inclusion (if the user name does not exist), another user can do the same action in same time?

Best Answer

Theoretically you could end up with duplicate usernames. In a scenario of high incoming requests, a race condition could occur.

If you want to avoid this, just put an UNIQUE index on your column. You could create a UNIQUE index on username by using this query:

CREATE UNIQUE INDEX IDX_USERNAME ON `user_tbl` (`username`);

Prior to creating this unique index, please verify that there are no duplicate entries in your table, as the index creation would fail.

When you have an unique index on this field, it will be impossible to end up with duplicate usernames. When a username is inserted, the table is locked during the insertion, this guarantees that a race condition would not occur.

Related Solutions

MySQL: Execute CREATE TEMPORARY TABLE SELECT just one time

Here is the nature of table creation with CREATE TEMPORARY TABLE

You can use the TEMPORARY keyword when creating a table. A TEMPORARY table is visible only to the current connection, and is dropped automatically when the connection is closed. This means that two different connections can use the same temporary table name without conflicting with each other or with an existing non-TEMPORARY table of the same name. (The existing table is hidden until the temporary table is dropped.) To create temporary tables, you must have the CREATE TEMPORARY TABLES privilege.

My guess would be you must have done the following sequence of events:

You ran CREATE TEMPORARY TABLE
Terminated DB Connection (which makes the temporary table drop immediately)
You had to run CREATE TEMPORARY TABLE again because he table did not exist

I have seen this problem victimize many developers, especially when it comes to MySQL Replication. For example, if you run STOP SLAVE;, the SQL Thread will close. Any tables create with CREATE TEMPORARY TABLE will be dropped. When you run START SLAVE;, MySQL Replication breaks instantly because the table needed for INSERTs, UPDATEs, or DELETEs no longer exist. I have had to perform mysqlbinlog dumps on relay logs to locate the original CREATE TEMPORARY TABLE statements, run them as CREATE TABLE in order for the table to remain accessible to other DB Connections, including the SQL Thread when START SLAVE; is reissued.

As for your Questions

Is there a way to tell MySQL that the creation of the temporary table and the SELECT query are just executed if the table does not exist?
Maybe with an if-condition?
Or can I check to some information schema if a temp-table exists?

You cannot check the information_schema for temp table existence. In the OS, there is no manifested .frm in datadir. You could find it in the folder defined in tmpdir.

Here is an example: I will create a temporary table called test.junk and try to locate the whereabouts of the table. (Note: I am using MySQL for Windows)

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.5.12-log MySQL Community Server (GPL)

Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> use test
Database changed
mysql> create temporary table junk (a int) engine=MyISAM;
Query OK, 0 rows affected (0.02 sec)

mysql> show create table junk\G
*************************** 1. row ***************************
       Table: junk
Create Table: CREATE TEMPORARY TABLE `junk` (
  `a` int(11) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1
1 row in set (0.01 sec)

mysql> select count(1) from information_schema.tables
-> where table_schema='test' and table_name='junk';
+----------+
| count(1) |
+----------+
|        0 |
+----------+
1 row in set (0.08 sec)

mysql> show variables like 'datadir';
+---------------+-----------------------+
| Variable_name | Value                 |
+---------------+-----------------------+
| datadir       | C:\MySQL_5.5.12\data\ |
+---------------+-----------------------+
1 row in set (0.00 sec)

mysql> show variables like 'tmpdir';
+---------------+--------------------------------------+
| Variable_name | Value                                |
+---------------+--------------------------------------+
| tmpdir        | C:\Users\redwards\AppData\Local\Temp |
+---------------+--------------------------------------+
1 row in set (0.00 sec)

mysql>

The table is not visible in INFORMATION_SCHEMA.TABLES. Let's locate the table in the OS

C:\MySQL_5.5.12\data\test>cd
C:\MySQL_5.5.12\data\test

C:\MySQL_5.5.12\data\test>dir junk.*
 Volume in drive C has no label.
 Volume Serial Number is 2C92-485B

 Directory of C:\MySQL_5.5.12\data\test

File Not Found

C:\MySQL_5.5.12\data\test>dir C:\Users\redwards\AppData\Local\Temp\*.frm
 Volume in drive C has no label.
 Volume Serial Number is 2C92-485B

 Directory of C:\Users\redwards\AppData\Local\Temp

10/19/2012  11:04 AM             8,554 #sql7ac_3_0.frm
               1 File(s)          8,554 bytes
               0 Dir(s)  190,200,049,664 bytes free

C:\MySQL_5.5.12\data\test>dir C:\Users\redwards\AppData\Local\Temp\*.MY*
 Volume in drive C has no label.
 Volume Serial Number is 2C92-485B

 Directory of C:\Users\redwards\AppData\Local\Temp

10/19/2012  11:04 AM                 0 #sql7ac_3_0.MYD
10/19/2012  11:04 AM             1,024 #sql7ac_3_0.MYI
               2 File(s)          1,024 bytes
               0 Dir(s)  190,200,049,664 bytes free

C:\MySQL_5.5.12\data\test>

There is the temp table, in the tmpdir folder. When I closed the mysql client and run dir C:\Users\redwards\AppData\Local\Temp\*.frm, the table vanished.

RECOMMENDATION

You can do one of two things

Create the temp table, load it with data, keep the same DB Connection around, and then access the data with the same ProcessID.
Do CREATE TEMPORARY TABLE IF NOT EXISTS (This is really a band-aid to improper design but may circumvent the problem).

UPDATE 2012-12-18 13:06 EDT

Further Observation #1

If you stayed with the same DB Connection and still have this problem, here something else not previously mentioned: You gave the temp table an index. That should be a UNIQUE INDEX or PRIMARY KEY.

Further Observation #2

You could shift the operation from a CREATE TABLE ... SELECT to CREATE TABLE ... INSERT IGNORE

Further Observation #3

If you want to use temp tables, please specify MyISAM. InnoDB has a tendency to leave pigeon holes in the data dictionary with ibdata1 in the event of a crash or upon DB Connection termination.

With these three(3) observations in mind, here is my recommended change

CREATE TEMPORARY TABLE IF NOT EXISTS cache
(id int(11) NOT NULL, PRIMARY KEY (id)) ENGINE=MyISAM;
INSERT IGNORE INTO cache SELECT id FROM table WHERE xyz;

CREATE TEMPORARY TABLE IF NOT EXISTS cache
(id int(11) NOT NULL, PRIMARY KEY (id)) ENGINE=MyISAM;
INSERT IGNORE INTO cache SELECT id FROM table WHERE xyz;

Give it a Try !!!

Mysql – Row locking or avoiding duplicate insertion on primarykey/otherfield combo

The OP answered their own question in the comments (now deleted):

Never mind I figured it out, using a unique index combo, ALTER TABLE files ADD UNIQUE INDEX (file_relation, file_relation_id, file_dimension) does the trick.