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. More information on setting the packet size is given in Section A.2.9, “Packet too large”.

You also get a lost connection if you are sending a packet 16MB or larger if your client is older than 4.0.8 and your server is 4.0.8 and above, or the other way around.

You may also see the MySQL server has gone away error if MySQL is started with the --skip-networking option.

You have encountered a bug where the server died while executing the query.

Indicate whether the MySQL server died. You can find information about this in the server error log. See Section A.4.2, “What to Do If MySQL Keeps Crashing”.

If a specific query kills mysqld and the tables involved were checked with CHECK TABLE before you ran the query, can you provide a reproducible test case? See Section E.1.6, “Making a Test Case If You Experience Table Corruption”.

What is the value of the wait_timeout system variable in the MySQL server? (mysqladmin variables gives you the value of this variable.)

Have you tried to run mysqld with the --log option to determine whether the problem query appears in the log?