Smart defaults [SSDT]

Posted by jamiet on SQL Blog See other posts from SQL Blog or by jamiet
Published on Thu, 31 May 2012 10:55:12 GMT Indexed on 2012/05/31 16:46 UTC
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I’ve just discovered a new, somewhat hidden, feature in SSDT that I didn’t know about and figured it would be worth highlighting here because I’ll bet not many others know it either; the feature is called Smart Defaults. It gets around the problem of adding a NOT NULLable column to an existing table that has got data in it – previous to SSDT you would need to define a DEFAULT constraint however it does feel rather cumbersome to create an object purely for the purpose of pushing through a deployment – that’s the situation that Smart Defaults is meant to alleviate.

The Smart Defaults option exists in the advanced section of a Publish Profile file:

image

The description of the setting is “Automatically provides a default value when updating a table that contains data with a column that does not allow null values”, in other words checking that option will cause SSDT to insert an arbitrary default value into your newly created NON NULLable column. In case you’re wondering how it does it, here’s how:

SSDT creates a DEFAULT CONSTRAINT at the same time as the column is created and then immediately removes that constraint:

ALTER TABLE [dbo].[T1]
   
ADD [C1] INT NOT NULL,
       
CONSTRAINT [SD_T1_1df7a5f76cf44bb593506d05ff9a1e2b] DEFAULT 0 FOR [C1];

ALTER TABLE [dbo].[T1] DROP CONSTRAINT [SD_T1_1df7a5f76cf44bb593506d05ff9a1e2b];

You can then update the value as appropriate in a Post-Deployment script. Pretty cool!

On the downside, you can only specify this option for the whole project, not for an individual table or even an individual column – I’m not sure that I’d want to turn this on for an entire project as it could hide problems that a failed deployment would highlight, in other words smart defaults could be seen to be “papering over the cracks”. If you think that should be improved go and vote (and leave a comment) at [SSDT] Allow us to specify Smart defaults per table or even per column.

@Jamiet

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