Add load command to import schema

[s1owjke]

I'm suggesting to add the ability to import a schema from file as is done in Active Record Migrations

Potential Benefits:

• No extra dependencies and simpler configuration, to setup environments for tests it's enough just to import the schema.sql, instead of adding extra utilities for specific database to do that
• Small performance boost on projects with a large number of migrations, not really relevant for production, but in ci/cd it can save some time

[dossy]

As I've never worked on a project where this was ever done, I don't have an opinion on how useful this kind of schema squashing really is. I'll let others who have experience with this functionality and its merits drive the review.

[amacneil]

I think there is quite a lot happening in this PR.

I would be in favor of adding a dbmate load command to load the schema.sql file, so you could use that in CI etc if you want. It's pretty much the same as running psql so maybe not super useful, but it also seems relatively straightforward.

Regarding "squashing migrations", my opinion is that you can already do that today. Just take the schema.sql contents, put that into a new migration, make sure you're using IF EXISTS guards in each of the statements, and delete the old migrations. Dbmate should be perfectly happy with you doing this - you will have some extra entries in your schema_migrations table but they won't affect anything.

I don't understand the other changes prune and --autoload-schema. I prefer we don't make any breaking changes to dbmate up or any new features that add complexity for people using the tool.

[s1owjke]

@amacneil I removed the squashing functionality completely, you are right that this can be achieved in another way, and if squashing happens in a project, it doesn't happen too often

Regarding the load schema - In ci/cd this should make flow a bit easier, e.g. mysql cli does not accept uri and expects a different format - separate host, port, login and password

Without additional flags in the current api it is not possible to achieve backwards compatibility, but I absolutely agree that there is no point in complicating the current one, let's keep it simple

[amacneil]

Sounds good!

@dossy want to do a code review here?

[amacneil]

@dossy want to re-review? then we can merge

[dossy]

LGTM now, thanks!