MySQL how to know the specific time when an error happens -

I have a table with 800 million lines now I try to add a colume to it:

< Pre> add optional table test_table mydata blob;

This command takes a very long time and finally returns an error:

  Error 2013 (HY000): connection to MySQL server during the query Lost  

I have to look at the MySQL log to see what's going on here. However, I do not know when this error will occur. Anyway, I can detect specific times when MySQL exits this error.

In addition to this, I am interested in why I get this error while doing a long-term query.