Question

I am currently playing around with MySQL and PHP, and I was wondering about table locks. From the MySQL documentation I know that MySQL applies table wide read locks, which can cause other queries to wait until the select query finishes.

When I am using PHP to query data like in this example:

$dbConnection = mysql_connect($dbHost, $dbUser, $dbPass) 
mysql_select_db($dbName, $dbConnection) 

$qry = "select * from bigTable";
$result = mysql_query($qry, $dbConnection);

//Read Data

mysql_close($dbConnection);

When, from the perspective of the PHP code, will the read lock get removed from “bigTable”?

Thank you,
Emi

Answer 1

The lock is released at the end of your read query:

$result = mysql_query($qry, $dbConnection);

because you are doing per-query transactions.

If you want to do more complex transactions, make sure that you use the innodb engine and start your transaction:

mysql_query("START TRANSACTION", $dbConnection)
mysql_query("BEGIN", $dbConnection);

Then do your work:

• for example read a table
• then write some values, etc.

and either commit or rollback the transaction:

mysql_query("COMMIT", $dbConnection);
mysql_query("ROLLBACK", $dbConnection);

depending if you want the changes to go through or not.