CREATE TABLE TestTab (ID INT IDENTITY(1,1), st nvarchar(100))
INSERT INTO TestTab (st) values ('a')
INSERT INTO TestTab (st) values ('b')
INSERT INTO TestTab (st) values ('c')
INSERT INTO TestTab (st) values ('d')
INSERT INTO TestTab (st) values ('e')
INSERT INTO TestTab (st) SELECT TOP 10000 st from testtab
GO 30
ALTER TABLE TestTab ADD newcol nvarchar(10) DEFAULT 'newcol'
UPDATE TestTab SET newcol = 'newcol' --6 sec
ALTER TABLE TestTab ADD newcol1 nvarchar(10) DEFAULT 'newcol1' NOT NULL
DROP TABLE TestTab
When I execute this test script, the ALTER
with UPDATE
takes 6 seconds which is understandable.
However, the ALTER
with the DEFAULT NOT NULL
executes instantaneously even on a much larger table. Is there any explanation on why this is instantaneous? On the physical disk, data still needs to be written to all rows right?
I tried looking at SET STATISTICS IO ON
and the Query plan, however those dont seem to be available for DDL operations.
Best Answer
Yes, adding a column with NOT NULL and a default doesn't actually write the values to all the rows at the time of the alter, so it is no longer a size-of-data operation. When you select from the table, the columns are actually materialized from sys.system_internals_partition_columns, which prevents all the values from having to be written (until they are changed). Note that this doesn't work for all data types and requires Enterprise Edition.
Remus Rusanu explains this in more detail here:
Also, for an
ALTER
at least, we still can't show you a plan because SQL Server doesn't produce one, but to see I/O, you can use SQL Sentry Plan Explorer.* This screen shot shows adding a column, c5, "online" as described above, and then another column, c6, "offline" because LOB types are not supported. You can see the I/O is mostly expressed as reads rather than writes, but what's more telling is the (invalid!)UPDATE
associated with the offline alter.If you don't have Enterprise Edition, both statements will have the secondary
UPDATE
attached (and the associated reads). (And if you use the free version of Plan Explorer, which doesn't get the full query call stack, you won't see the above - you will just see an empty statement tree. A paid version is required to see the full query call stack.)Note that SQL Server will produce an estimated plan, but it's not very useful. At all. And the estimated plan for an online alter is identical to the estimated plan for an offline alter.
*Disclaimer: I work for SQL Sentry.