How to Fix Getting Errcode 28 on MySQL?

Query3 failed: Error writing file '/tmp/MY9uc2x7' (Errcode: 28)

If you are seeing this error, it is not a MySQL Error Code, but a Linux OS Error Code.

See the full message with the C++ debugging tool, perror:

perror 28

And this will output:

OS error code 28: No space left on device

So the problem is easily solved! Delete some stuff on the server hosting your DB.