Duplicate key in sysclsobjs using DBCC CLONEDATABASE

This blog post is about an error message I got the other day when using DBCC CLONEDATABASE in a T-sql-script. But first some background to DBCC CLONEDATABASE.

I was pretty excited about the DBCC CLONEDATABASE command, which was introduced in SQL Server 2014 SP2 and SQL Server 2016 SP1. It creates a schema-only (that means all the database objects, but no data) copy of a database, keeping all statistics data, so that you can troubleshoot Query plans for certain queries without having to copy all the data. Before DBCC CLONEDATABASE (and to be honest probably also afterwords, DBCC CLONEDATABASE doesn’t replace all the needs) one had to make a full copy of a database to get the statistics data along. That’s usually copied to a test box. If the test box is identical to your production box, you’re almost fine. But on your test box, you don’t have the cached execution plans from the production box. Therefore, you might end up with very different Query plans in your test box. With DBCC CLONEDATABASE, you get a readonly copy of a database, on your production box and you can use that to tweak your queries and see what new estimated execution plans they get.

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