Change Data Types large Live table
Summary

For large tables, it is nearly always preferable to move the data instead of altering the table. To get maximum speed, use one of the following patterns. Let us call the table you want to change S (for source).


Create an empty copy of S. Let us call that T
ALTER TABLE on T. This is very fast because T has no rows
If T has a cluster index, remove all other indexes but the cluster
If T is a heap, remove all indexes
Put the database in SIMPLE recovery mode (if you can, but be aware of the backup implications)
Use one of the two options provided in the Copy Data section below
Rebuild the indexes on T
Drop S and rename to T




Copy Data

In step 6 above, you will need to move data from S to T at highest possible speed. The following are two options:


Use INSERT ... SELECT
Use SSIS


Of these two, SSIS is the fastest. With INSERT ... SELECT you are restricted to a single thread. However, INSERT ... SELECT is much easier. See below for more details.

Option 1: INSERT ... SELECT

If T is a heap, do this:

INSERT INTO T WITH (TABLOCK) SELECT ... FROM S


This is minimally logged.

If T has a clustered index, do this:

DBCC TRACEON (610)
INSERT INTO T SELECT ... FROM S


This is also be minimally logged. The problem with this option is that you can only run one INSERT statement in parallel. Because SQL Server does not do parallel INSERT ... SELECT, this restricts you to around 40-80MB/sec - which is quite slow.

Option 2: Use SSIS

This option is a lot more trouble, but also faster. The idea is this:


Drop all indexes on T
Create an SSIS package that takes a parameter that selects a subset of the rows in S (using some filter that is supported by an index). The SSIS package then moves the rows to T using the TABLOCK hint in bulk mode
Execute multiple copies of the SSIS package, each operating on their own, distinct subset of S (so you don't duplicate rows in T)


This is MUCH faster because the packages can run in parallel. With this technique, I can typically move tables at GB/sec.