My problem is MySQL runs into a 'Too many connections' error when I add a new column to my live database.

The database has 1 million records so it takes a while to add this. 

* What is the reason the table locks?
* What to do to keep table running?

The query I execute on the table with 1 million records (InnoDB)

```
ALTER TABLE table_name 
ADD COLUMN new_table_column INT(11) NULL 
DEFAULT NULL ;

```

Thanks!