Adding a new Big INT column to existing table in production, which holds 700 million records will impact anything in production?

Question

Hi Guys,
I have to add a new Big INT column to existing table in production, which holds 700 million records and would like to know the impact?
I have been tolled by one of my colleagues that last time they tried adding a column to same table during working hour and it locked out the table and impacted the users.
Please suggest/share If any one had similar experience.
Thanks Shiven:) If Answer is Helpful, Please Vote

Answer 1

If you add a new column to a table using an ALTER TABLE ADD command and specify that the new column allows NULLs and you do not define a default value, then it will take a table lock.  However, once it gets the table lock, it will essentially run instantly
and then free the table lock.  That will add this new column as the last column in the table, for example
ALTER MyTable ADD MyNewColumn bigint NULL;
But if you your change adds a new column with a default value, or you do something like using table designer to add the new column in the middle of the current list of columns, then SQL will have to rewrite the table.  So it will get a table lock, rewrite
the whole table and then free the table lock.  That will take a considerable amount of time and the table lock will be held for that whole period of time.
But, no matter how you make the change, if at all possible, I would not alter a table schema on a production database during working hours.  Do it when nothing else is going on.
Tom