Reducing size of datatype involved in index fails where as increasing size work

Question

This question is related to my question in SO Database
events that will cause deletion of index
For example if i create a table like
CREATE TABLE dbo.Customers (ID INT IDENTITY(1,1) PRIMARY KEY CLUSTERED, CustomerName NVARCHAR(200));
CREATE INDEX IX_CustomerName ON dbo.Customers(CustomerName);
And then try to change column datatype like shown below to 230 from 200.
alter table Customers
alter column CustomerName nvarchar(230) null
The above alter command runs successfully and index IX_CustomerName is not dropped.
But if i try to reduce the size of column to 150 from 200 then it shows error.
alter table Customers
alter column CustomerName nvarchar(150) null
I get error like
Msg 5074, Level 16, State 1, Line 1
The index 'IX_CustomerName' is dependent on column 'CustomerName'.
Msg 4922, Level 16, State 9, Line 1
ALTER TABLE ALTER COLUMN CustomerName failed because one or more objects access this column.
So reducing the size(data may loss) of a column does not work if index is present on a column where as if the index not present then above
query works successfully. So why does it does not allow reducing the column when index present where as increasing size does not show any problem and works?

Answer 1

Hello,
Indeed, a little bit strange even because in BOL it's documented for a different behaviour, see
ALTER TABLE (Transact-SQL) => Remarks =>
"Changing the Size of a Column
You can change the length, precision, or scale of a column by specifying a new size for the column data type in the ALTER COLUMN clause. If data exists in the column, the new size cannot be smaller than the maximum size of the data. Also, the column cannot
be defined in an index, unless the column is a varchar, nvarchar, or varbinary data type and the index is not the result of a PRIMARY KEY constraint. See example P."
Olaf Helper
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