create primary key on existing table with data - sql

Question

As part of a migration project, we have imported data from a JDE iSeries DB2 database. An SSIS package was created to create the destination tables and import data. The import went successfully.
Now comes the problem - The customer wants Primary Keys created in the destination DB (SQL 2008 R2). The problem table in this case, would be one table that has 104 columns and 7.5 million rows of data. The PK required for this table is composite and has 7 columns.
We are considering this :
BEGIN TRANSACTION
GO
ALTER TABLE [dbo].[F0911] ADD CONSTRAINT [F0911_PK] PRIMARY KEY CLUSTERED
(
[GLDCT] ASC,
[GLDOC] ASC,
[GLKCO] ASC,
[GLDGJ] ASC,
[GLJELN] ASC,
[GLLT] ASC,
[GLEXTL] 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) ON [PRIMARY]
GO
COMMIT
or this:
-- Rename existing tables
sp_RENAME '[F0911]' , '[F0911_old]'
GO
-- Create new table
SELECT * INTO F0911 FROM F0911_old WHERE 1=0
GO
--Create PK constraints
ALTER TABLE [dbo].[F0911] ADD CONSTRAINT [F0911_PK] PRIMARY KEY CLUSTERED
(
[GLDCT] ASC,
[GLDOC] ASC,
[GLKCO] ASC,
[GLDGJ] ASC,
[GLJELN] ASC,
[GLLT] ASC,
[GLEXTL] 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) ON [PRIMARY]
GO
--Insert data into new tables
INSERT INTO F0911
SELECT * FROM F0911_old
GO
-- Drop old tables
DROP TABLE F0911_old
GO
Which would be a more efficient approach, performance wise? I have a gut feeling that both are the same and even the first approach does the same thing as the second one does, implicitly. Is this understanding correct?
Please note that all these columns already exist in the table and we cannot modify the table definition.
Thanks,
Raj

Answer 1

They're the same. The effect of creating a clustered index is to arrange the pages which will happen in both cases. For non-clustered indexes it will help to disable the index and then turn it back on and rebuilding it.

Answer 2

I think the first approach is right, but I don't understand the reason of BEGIN Transaction and END transaction. I don't think Transaction keyword is necessary because you are not modifying data of the table. Transaction is used where we have to lock the data and we are modifying real time data so the old data is not used.