How do you add a NOT NULL Column to a large table in SQL Server?

Question

To add a NOT NULL Column to a table with many records, a DEFAULT constraint needs to be applied. This constraint causes the entire ALTER TABLE command to take a long time to

Answer 1

I would use CURSOR instead of UPDATE. Cursor will update all matching records in batch, record by record -- it takes time but not locks table.

If you want to avoid locks use WAIT.

Also I am not sure, that DEFAULT constrain changes existing rows. Probably NOT NULL constrain use together with DEFAULT causes case described by author.

If it changes add it in the end So pseudocode will look like:

-- without NOT NULL constrain -- we will add it in the end
ALTER TABLE table ADD new_column INT DEFAULT 0

DECLARE fillNullColumn CURSOR LOCAL FAST_FORWARD
    SELECT 
        key
    FROM
        table WITH (NOLOCK)
    WHERE
        new_column IS NULL

OPEN fillNullColumn

DECLARE 
    @key INT

FETCH NEXT FROM fillNullColumn INTO @key

WHILE @@FETCH_STATUS = 0 BEGIN
     UPDATE
         table WITH (ROWLOCK)
     SET
         new_column = 0 -- default value
     WHERE
         key = @key

     WAIT 00:00:05 --wait 5 seconds, keep in mind it causes updating only 12 rows per minute

     FETCH NEXT FROM fillNullColumn INTO @key
END

CLOSE fillNullColumn
DEALLOCATE fillNullColumn

ALTER TABLE table ALTER COLUMN new_column ADD CONSTRAIN xxx

I am sure that there are some syntax errors, but I hope that this help to solve your problem.

Good luck!