Errors when reducing VARCHAR length - sql

Question

Please see the DDL below:
CREATE TABLE Person
(
ID int identity not null,
Name VARCHAR(100),
Age int,
EyeColour varchar(20),
primary key (ID)
)
CREATE INDEX Name ON Person (Name)
CREATE INDEX Age ON Person (Age)
CREATE INDEX EyeColour ON Person (EyeColour)
I can execute the following statement:
ALTER TABLE person
ALTER COLUMN name VARCHAR(110)
However, I cannot execute the following statement:
ALTER TABLE person
ALTER COLUMN name VARCHAR(90)
The error is:
Msg 5074, Level 16, State 1, Line 1
The index 'Name' is dependent on column 'name'.
Msg 4922, Level 16, State 9, Line 1
ALTER TABLE ALTER COLUMN name failed because one or more objects access this column.
Why am I seeing these errors when I reduce the length of a VARCHAR. In what other scenarios would I see this error e.g. change a data type?
Is there an automated way of identifying all the indexes and constraints affected by a data type change and dealing with them?

Answer 1

You could definitely automate the identification part. Here is a script that will find all the indices and foreign keys depending on a table column (I have left finding check constraints as an exercise for the reader). As it is a script, you would have to change the parameters when you run it. You could also turn this into a stored procedure.
However, I don't recommend automating the actions such as dropping indices or constraints. You are better off reviewing the output and deciding if it is really OK to reduce the sizes of the columns or not in the light of what you know about the applications that use these data.
-- parameters
DECLARE
#nm_schema sysname = N'Purchasing',
#nm_table sysname = N'PurchaseOrderHeader',
#nm_column sysname = N'EmployeeID';
DECLARE
#id_table int,
#id_column smallint;
SELECT #id_table = o.object_id
FROM sys.objects o
JOIN sys.schemas s
ON o.schema_id = s.schema_id
WHERE o.name = #nm_table
AND s.name = #nm_schema
AND o.type = 'U';
IF ##ROWCOUNT = 0
BEGIN
RAISERROR(N'Schema %s table %s not found', 0, 1, #nm_schema, #nm_table);
RETURN;
END;
SELECT #id_column = column_id
FROM sys.columns
WHERE object_id = #id_table
AND name = #nm_column;
IF ##ROWCOUNT = 0
BEGIN
RAISERROR(N'Column %s not found in schema %s table %s', 0, 2, #nm_column, #nm_schema, #nm_table);
RETURN;
END;
SELECT 'Index' AS 'dependency', i.name
FROM sys.indexes i
JOIN sys.index_columns ic
ON i.object_id = ic.object_id
AND i.index_id = ic.index_id
WHERE i.object_id = #id_table
AND ic.column_id = #id_column
AND NOT i.type = 0 -- heap
UNION ALL
SELECT 'FKey', f.name
FROM sys.foreign_keys f
JOIN sys.foreign_key_columns fc
ON f.object_id = fc.constraint_object_id
WHERE (f.parent_object_id = #id_table AND fc.parent_column_id = #id_column)
OR (f.referenced_object_id = #id_table AND fc.referenced_column_id = #id_column);