what is the issue with database changes?

[Matt Mackall]

On Tue, 2015-03-03 at 15:24 +0100, Thomas De Schampheleire wrote:
> Hi,
> 
> Regularly I hear that we don't want to change the model yet to stay
> backwards compatible.
> 
> However, I do see several 'dbmigrate' scripts in the source base,
> which hint at it being possible to migrate across database changes.
> 
> Can someone explain in more detail why we do not want such database
> changes (yet)?

On-disk format migrations are an anti-pattern of software development.
They're fragile and one-way and present a large barrier to user
acceptance of new versions. And there's no excuse for them when you have
a database rather than a file format: you can always add new tables
without changing the structure of the existing tables.

-- 
Mathematics is the supreme nostalgia of our time.