Question

We need to change the data types of some columns from int to bigint. Unfortunately some of these tables are large, around 7-10 million rows (but not wide).

Alter table alter column is taking forever on these tables. Is there a faster way to achieve this?

Answer 1

Coincidentally, I had to do something very similar about 3 hours ago. The table was 35m rows, it is fairly wide, and it was taking forever to just do this:

alter table myTable add myNewColumn int not null default 0;

Here’s what what I ended up going with:

alter table myTable add myNewColumn int null;

while 1=1
begin
    update top (100000) myTable
    set
        myNewColumn = 0
    where
        myNewColumn is null;

    if @@ROWCOUNT = 0 break;
end

alter table myTable alter column myNewColumn int not null;
alter table myTable add constraint tw_def_myNewColumn default (0) for myNewColumn;

This time around, the alter table statements were near-instant. It took about 7-8 minutes (on a slow server) to do the update batches. I’m speculating that SQL Server was generating undo in my original query to restore the values, but I didn’t expect that starting off.

Anyway, in your case, maybe something similar would help. You could try adding a new bigint column, update the new column in batches, then set the constraints on it.