Change a Primary Key from Nonclustered to Clustered

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Suppose I have an SQL Server 2005 table, TableX, with 2 indexes on it:

PK_TableX = PRIMARY KEY NONCLUSTERED on FieldA
IX_TableX_FieldB = CLUSTERED on FieldB

I want to switch the PK to be CLUSTERED, and the other index to be NONCLUSTERED.

I have to assume that the database will be in use at the moment I try to change the indexes round - so my primary concern that I want to avoid, is that at some point in the process the PK constraint will not exist on the table. I want to be protected against any risk of duplicate keys being inserted.

i.e. I can't just drop the primary key and recreate it.

This process needs to be done via an SQL script, not via SSMS.

I have an approach which I think will work (I'll post it as a potential answer), but would like to open it up in case I'm missing something or there is another/better way. Plus, it may prove useful for others in the future

indexing sql-server-2005 sql-server sql
4 Answers

1) Drop the existing clustered index first (IX_TableX_FieldB):

   DROP INDEX TableX.IX_TableX_FieldB

2) Create a (temporary) UNIQUE constraint on the unique fields referenced in the primary key

    ALTER TABLE TableX
    ADD CONSTRAINT UQ_TableX UNIQUE(FieldA)

3) Drop the PRIMARY KEY

    ALTER TABLE TableX
    DROP CONSTRAINT PK_TableX

4) Recreate the PRIMARY KEY as CLUSTERED

   ALTER TABLE TableX
   ADD CONSTRAINT PK_TableX PRIMARY KEY CLUSTERED(FieldA)

5) Drop the temporary UNIQUE constraint

   ALTER TABLE TableX
   DROP CONSTRAINT UQ_TableX

6) Add the IX_TableX_FieldB back on as NONCLUSTERED

   CREATE NONCLUSTERED INDEX IX_TableX_FieldB ON TableX(FieldB)
user_1355310

Here's a shortcut to overwrite the original key:

CREATE UNIQUE CLUSTERED INDEX [pk_name] ON [table_name]([id])
WITH DROP_EXISTING

And as mentioned in other answers you will still need to drop/re-add foreign constraints if present.

user_12884730

Adding double quotes to Jerloldrartolf's answer

IF  EXISTS (SELECT * FROM sys.tables WHERE object_id = OBJECT_ID(N'[dbo].[FKAgainstTableList]'))
BEGIN 
    DROP TABLE FKAgainstTableList
END
--CREATE TABLE FKAgainstTableList (ForeignKey VARCHAR(30),[Table] VARCHAR(30))
DECLARE @PKTableName VARCHAR(100), 
        @PKName varchar(100),
        @FKName varchar(100),
        @sql varchar(max),
        @PKcolumnName varchar(30),
        @table VARCHAR(100),
        @FKColumnName VARCHAR(100), 
        @parentColumnNumber int
SET @PKTableName = 'mytable'
set @PKName = (SELECT name FROM sys.indexes WHERE OBJECT_NAME(object_id) = @PKTableName AND is_primary_key = 1)
set @PKcolumnName = (SELECT name FROM sys.columns WHERE OBJECT_NAME(object_id) = @PKTableName AND is_identity =1)
PRINT @PKcolumnName

 SELECT  OBJECT_NAME(sys.foreign_key_columns.parent_object_id) [Table],sys.columns.name [FKColumnName],sys.foreign_keys.name [FKName] 
    INTO FKAgainstTableList
    FROM sys.foreign_keys INNER JOIN sys.foreign_key_columns 
    ON sys.foreign_keys.object_id = sys.foreign_key_columns.constraint_object_id
    INNER JOIN sys.columns ON sys.columns.object_id = sys.foreign_keys.parent_object_id AND sys.columns.column_id = sys.foreign_key_columns.parent_column_id
    WHERE OBJECT_NAME(sys.foreign_keys.referenced_object_id) = @PKTableName


DECLARE table_cur1 CURSOR  FOR
    SELECT  * FROM FKAgainstTableList

    PRINT @sql


-------------------------------Disable constraint on FK Tables
OPEN table_cur1
FETCH NEXT FROM table_cur1 INTO @table,@FKColumnName,@FKName
WHILE   @@FETCH_STATUS = 0
    BEGIN
        SET @sql ='ALTER TABLE '+@table+' DROP CONSTRAINT "'+ @FKName+'"'
        PRINT @sql
        FETCH NEXT FROM table_cur1 INTO @table,@FKColumnName,@FKName
    END
CLOSE table_cur1
DEALLOCATE table_cur1

--------------------------------DROP AND recreate CLUSTERED pk
IF  EXISTS (SELECT 1 FROM sys.indexes WHERE object_id = OBJECT_ID(@PKTableName) AND name = @PKName)
BEGIN
    SET @sql = 'ALTER TABLE '+@PKTableName+' DROP CONSTRAINT "'+ @PKName +'"'
    PRINT @sql

END
SET @sql = 'ALTER TABLE '+@PKTableName +' ADD  CONSTRAINT '+@PKName+' PRIMARY KEY CLUSTERED ('+@PKcolumnName+' ASC)
WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 80) ON [PRIMARY]'
PRINT(@sql)
--------------------------------Enable FK constraints on FK tables.
DECLARE table_cur2 CURSOR  FOR
    SELECT  * FROM FKAgainstTableList
OPEN table_cur2
FETCH NEXT FROM table_cur2 INTO @table,@FKColumnName,@FKName
WHILE   @@FETCH_STATUS = 0
    BEGIN
        SET @sql = 'ALTER TABLE '+@table+' WITH NOCHECK ADD  CONSTRAINT  "'+ @FKName+'" FOREIGN KEY(['+@FKColumnName+'])
        REFERENCES ['+@PKTableName+'] (['+@PKcolumnName+'])'
        PRINT(@sql)
        SET @sql = 'ALTER TABLE '+@table+' CHECK CONSTRAINT  "'+@FKName+'"'
        PRINT(@sql)

        FETCH NEXT FROM table_cur2 INTO @table,@FKColumnName,@FKName

         END
CLOSE table_cur2
DEALLOCATE table_cur2
DROP TABLE FKAgainstTableList

I found I needed to run the generated script in 3 parts

with generation from this part being the middle part.

SET @sql = 'ALTER TABLE '+@PKTableName +' ADD  CONSTRAINT '+@PKName+' PRIMARY KEY CLUSTERED ('+@PKcolumnName+' ASC)
WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 80) ON [PRIMARY]'
PRINT(@sql)
user_15928210
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