The most common reason for the MySQL server has gone away error is that the server timed out and closed the connection. In this case, you normally get one of the following error codes (which one you get is operating system-dependent):
Error Code Description
CR_SERVER_GONE_ERROR The client couldn't send a question to the server.
CR_SERVER_LOST The client didn't get an error when writing to the server, but it didn't get a full answer (or any answer) to the question.
By default, the server closes the connection after eight hours if nothing has happened. You can change the time limit by setting the wait_timeout variable when you start mysqld.
Some other common reasons for the MySQL server has gone away error are:
You (or the db administrator) has killed the running thread with a KILL statement or a mysqladmin kill command.
You tried to run a query after closing the connection to the server. This indicates a logic error in the application that should be corrected.
You got a timeout from the TCP/IP connection on the client side. This may happens if you have been using the commands: mysql_options(..., MYSQL_OPT_READ_TIMEOUT,...) or mysql_options(..., MYSQL_OPT_WRITE_TIMEOUT,...). In this case increasing the timeout may help solve the problem.
You have encountered a timeout on the server side and the automatic reconnection in the client is disabled (the reconnect flag in the MYSQL structure is equal to 0).
You are using a windows client and the server had dropped the connection (probably because wait_timeout expired) before the command was issued.
The problem on windows is that in some cases MySQL doesn't get an error from the OS when writing to the TCP/IP connection to the server, but instead gets the error when trying to read the answer from connection.
In this case, even if the reconnect flag in the MYSQL structure is equal to 1, MySQL does not automatically reconnect and re-issue the query as it doesn't know if the server did get the original query or not.
The solution to this is to either do a mysql_ping on the connection if there has been a long time since the last query (this is what MyODBC does) or set wait_timeout on the mysqld server so high that it in practice never times out.
You can also get these errors if you send a query to the server that is incorrect or too large. If mysqld receives a packet that is too large or out of order, it assumes that something has gone wrong with the client and closes the connection. If you need big queries (for example, if you are working with big BLOB columns), you can increase the query limit by setting the server's max_allowed_packet variable, which has a default value of 1MB. You may also need to increase the maximum packet size on the client end.
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