Since you're only doing INSERT and not UPDATE, you don't need MERGE.

I really can't follow the different tables you have since there's only one source table for all the columns...

You'll need to CAST any text column(s) to varchar(max) in the PARTITION BY.

;WITH cte_count_dups AS (
    SELECT 
        *, COUNT(*) OVER(PARTITION BY Nationalnumber, Birthdate, Mobilphone, Emailaddress, Firstname) AS dup_count
...

Not a lot of details.  Maybe something like this:

SELECT 
    NationalNumber
FROM Contact
GROUP BY NationalNumber
HAVING /*COUNT(*) > 1 AND*/
    ((COUNT(DISTINCT Birthdate) >...

You can't include any columns in a clustered index (by definition it automatically includes all columns in the table anyway).

Just use the key column(s) you need.  It's OK, and often...

The DBA's right about the concurrency issue.  Also, you should really only create a clustered index on a temp table and it should be created before the table...

The DBA's right about the concurrency issue.  Also, you should really only create a clustered index on a temp table and it should be created before the table is loaded.

You...

You can reference a temp table in dynamic SQL.  Could you create a temp table and load it instead?

SELECT j.name AS running_job_name, 
    DATEDIFF(MINUTE, ja.start_execution_date, GETDATE()) AS duration_mins,
    ja.* 
FROM msdb.dbo.sysjobactivity ja
INNER JOIN msdb.dbo.sysjobs j ON j.job_id = ja.job_id
WHERE...

> Two synchronous replica in one region <<

The prod server must wait for the replicas to be in sync.  That could also be adding to delay in prod.

Typically you create only the clustered index prior to load, and any non-clustered indexes after the load.  That's generally the best-performing approach, although theoretically it could be different for a...

It's 16 Gb. My laptop from 10-11 years ago had double that amount of ram. My phone from 2010ish with its 480x800 resolution screen had that amount...

It's 16 Gb. My laptop from 10-11 years ago had double that amount of ram. My phone from 2010ish with its 480x800 resolution screen had that amount...

It's 16 Gb. My laptop from 10-11 years ago had double that amount of ram. My phone from 2010ish with its 480x800 resolution screen had that amount of storage.

Maybe...

A PK is not necessarily an irrelevant cost.  If I have 2B rows, for example, the "free" PK costs me at least 16GB of row and storage space.

Some tables simply...