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 run if the table is very large. This is because:

Assumptions:

1. The DEFAULT constraint modifies existing records. This means that the db needs to increase the size of each record, which causes it to shift records on full data-pages to other data-pages and that takes time.
2. The DEFAULT update executes as an atomic transaction. This means that the transaction log will need to be grown so that a roll-back can be executed if necessary.
3. The transaction log keeps track of the entire record. Therefore, even though only a single field is modified, the space needed by the log will be based on the size of the entire record multiplied by the # of existing records. This means that adding a column to a table with small records will be faster than adding a column to a table with large records even if the total # of records are the same for both tables.

Possible solutions:

1. Suck it up and wait for the process to complete. Just make sure to set the timeout period to be very long. The problem with this is that it may take hours or days to do depending on the # of records.
2. Add the column but allow NULL. Afterward, run an UPDATE query to set the DEFAULT value for existing rows. Do not do UPDATE *. Update batches of records at a time or you’ll end up with the same problem as solution #1. The problem with this approach is that you end up with a column that allows NULL when you know that this is an unnecessary option. I believe that there are some best practice documents out there that says that you should not have columns that allow NULL unless it’s necessary.
3. Create a new table with the same schema. Add the column to that schema. Transfer the data over from the original table. Drop the original table and rename the new table. I’m not certain how this is any better than #1.

Questions:

1. Are my assumptions correct?
2. Are these my only solutions? If so, which one is the best? I f not, what else could I do?

Answer 1

I ran into this problem for my work also. And my solution is along #2.

Here are my steps (I am using SQL Server 2005):

1) Add the column to the table with a default value:

ALTER TABLE MyTable ADD MyColumn varchar(40) DEFAULT('')

2) Add a NOT NULL constraint with the NOCHECK option. The NOCHECK does not enforce on existing values:

ALTER TABLE MyTable WITH NOCHECK
ADD CONSTRAINT MyColumn_NOTNULL CHECK (MyColumn IS NOT NULL)

3) Update the values incrementally in table:

GO
UPDATE TOP(3000) MyTable SET MyColumn = '' WHERE MyColumn IS NULL
GO 1000

• The update statement will only update maximum 3000 records. This allow to save a chunk of data at the time. I have to use “MyColumn IS NULL” because my table does not have a sequence primary key.

• GO 1000 will execute the previous statement 1000 times. This will update 3 million records, if you need more just increase this number. It will continue to execute until SQL Server returns 0 records for the UPDATE statement.