Question

Preamble

I was modifying a column in SQL Server 2008 today, changing the datatype from something like currency(18,0) to (19,2).

I got the error “The changes you have made require the following tables to be dropped and re-created” from SQL Server.

Before you scramble to answer, please read the following:

I already know there is the option in Tools ► Options ► Designer ► Table and Database Designers ► Uncheck the box “Prevent saving changes that require table re-creation.”

…so do not answer with that!

Actual question

My actual question is for something else, as follows:

Are there any negative effects / possible drawbacks of doing this?

Does the table actually get dropped and recreated automatically when this box is unchecked?

If so, is the table copy a 100% exact replica of the source table?

Answer 1

The table is only dropped and re-created in cases where that’s the only way SQL Server’s Management Studio has been programmed to know how to do it.

There are certainly cases where it will do that when it doesn’t need to, but there will also be cases where edits you make in Management Studio will not drop and re-create because it doesn’t have to.

The problem is that enumerating all of the cases and determining which side of the line they fall on will be quite tedious.

This is why I like to use ALTER TABLE in a query window, instead of visual designers that hide what they’re doing (and quite frankly have bugs) – I know exactly what is going to happen, and I can prepare for cases where the only possibility is to drop and re-create the table (which is some number less than how often SSMS will do that to you).

Are there any negative effects / possible drawbacks of doing this?

Sure. If you can script the change yourself without rebuilding the whole table, that’s better – consider the case where the table is 10TB, and the database is heavily logged (think sync AG, change tracking, replication, poorly-written triggers), and the table is highly accessed – that’s a potential recipe for disaster. If your change is something where you can apply an ONLINE hint or add a column and copy the data over in batches instead of all-or-nothing the GUI will do, this is better.

Does the table actually get dropped and recreated automatically when this box is unchecked?

It might. There is a laundry list of scenarios and the outcome is dependent on the version of SSMS, the version of SQL Server, and sometimes the edition. You can check by checking the box and trying to apply the change on a meaningless copy of the database first, but using actual ALTER TABLE scripts instead of the pointy-clicky GUI is the way to go IMHO.

If so, is the table copy a 100% exact replica of the source table?

Yes, if SSMS has to rebuild the table, it will be a 100% exact replica after it’s done (except for the change of course), but that could be next Wednesday. The process creates a new version of the table, copies all the data to it, then drop the old table and renames the new one.