Briefly,

1. Let me try with a simple example. Suppose you have a fact table A with fields (ItemID, Item, Amt) where Amt is in USD. Rule of thumb is: don't expose A to the consumer; instead write a SQL View V_A and expose that instead:

  CREATE VIEW V_A AS SELECT ItemID, Item, Amt FROM A

Then suppose a European counterpart wants to use the same API but needs the amounts to be in Euros. You can write another view: (in practice the conversion 0.92 shouldn't be a static number, this is just for illustration)

  CREATE VIEW V_A_EURO AS SELECT ItemID, Item, Amt * 0.92 AS AmtEUR FROM V_A

Expose this to the Europeans. You can keep stacking views on top of other views. Your U.S. consumers will always see the data through the lens of V_A and your European consumers will always see it through V_A_Euro.

Suppose the underlying table A now changes. There's been a merger and the company now stops reporting currencies in USD, and everything is now in British Pounds so your DBA adds a field AmtGBP and starts populating that field instead. Amt still contains historical data, but moving forward the data in Amt will be NULLs; AmtGBP is the new internal baseline currency. From a VIEW perspective, all you have to do is:

  ALTER VIEW V_A AS SELECT ItemID, Item, ISNULL(Amt, AmtGBP * 1.23) AS Amt FROM A

Your V_A and V_A_EURO consumers (could be Tableau, Excel, other SQL views, etc.) will still happily receive data per usual, unaware of the internal changes (the British are coming!) that have occurred. Contract kept.

  Table A <- View V_A <- View V_A_Euro

2. Cross cutting concerns come in many forms so not sure if I can address. Stored Procedures are definitely an acceptable abstraction -- they accept parameters and can return tabular results just like VIEWs. They do however work in a procedural manner (like subroutines) and can produce side effects, which is sometimes necessary to accomplish very specific tasks. VIEWs on the other hand are more similar to pure functions (unless random number generation is involved) with no side effects. Because views are dynamic, they flex with your data and VIEW definitions.

There's another step that could be added there, too: After the ALTER VIEW, V could be slowly incrementally updated over however long you need to back-populate AmtGBP, and the views will continue to just work the whole time. Once done, V_A can be simplified to remove the ISNULL and Amt, then Amt dropped from V. That way you don't get build-up of cruft over the years, and the experience isn't interrupted for the migration.

(Possibly a bad idea for currency conversion for various reasons, but just wanted to mention it since this type of migration may be just right for other data)

Is there anything you recommend for handling SQL definitions in version control, development and production envs?

For production, I created a command on the app that loads the stored procedures into the DB idempotently on each deployment/configuration. This won’t work if the app server scales but allowed us to store stored procs in VC.

For development, we ran the command on each page load as a sort of hacky “live reload”. It didn’t work well (which highlighted the issue with scalability in production) because Postgres, fairly, doesn’t like parallel redefinitions of the same stored proc.

I’m not sure how best to automate this. For production, seems like a case of running a command once per DB server.

And in development, using a fs watcher that loads changes in.

But I don’t know, this is new territory for us and I couldn’t find anything out there to manage it within the context of a web framework. Perhaps I’m searching for the wrong thing.

Web frameworks like Rails/Django use the idea of migrations to make changes to the database. The idea is that you have a set of migration scripts like: migrations/1765_create_table_users.sql migrations/2891_store_procedure_x.sql migrations/5892_change_store_procedure_x.sql

(.sql/.rb/.py, it doesn't matter).

And you have a "migrations" table in your database that contains the numbers of the migrations that have been run:

  select * from migrations;
      version
  ----------------
   1765
   2891

Every time you deploy to production automatically check which scripts in your db/migrations folder don't exist in the migrations table and run them. (In the current example, you'd run the 5892_change_store_procedure_x.sql that hasn't been run yet).

How to do with functions/procedures?

You commit the function definitions in a functions folder to your version system like:

  db/functions/report_x.sql
  CREATE or REPLACE function report_x() returns ...

When you change this file, nothing happens, you need to create a migration to re-run this code once. In rails migrations would be:

  class UpdateReportXFun < ActiveRecord::Migration[5.2]
    def up
      execute File.read(
        Rails.root.join('db','functions','report_x.sql')
      )
    end
  end

Yeah, I’m aware of that, thank you. I was wondering if there was a way with a faster feedback loop and allowed for bug fixes without creating a new migration.

You don't need to write the migration until you're done. It's possible to have a very tight feedback loop in any case.

I'm doing a lot of work in a Rails codebase where I edit views/functions/procedures all the time. My setup is quite usable.

My current setup: I edit those .sql files and run them with psql in my local while developing (without writing any migration yet).

I have some like this running on one screen to make sure the modified files are executed by psql immediately as I change them (you could use `guard` too):

  find ~/projectx/db/functions -type f -name "*.sql" | entr -d -p psql db_name -f /_

and I edit the db/functions/*.sql files freely, adding things, changing behaviour of functions and they are updated on the fly. (I can run tests -or try things in the browser- to verify my changes work as I expect).

--

Once I finish and I know everything is great, I just add the migration. The migration is simply an indicator of which files I've modified and to specify the right order to run them (which is useful if they are dependencies), like:

  # migration
  def up
    execute File.read(function1_sql_file)
    execute File.read(function2_sql_file)
  end

I could have an alias that automates generating that migration but it's just 4 lines...

[ I'm also using pgTAP to write tests for functions, it's quite nice :) ]

Oh wow, now I see what you mean. Thank you! That’s work great. I wasn’t aware of ‘entr’ either, that’s exactly what I had in mind!

I’ll have a look at pgTAP too. Naturally we want to test in CI, I can see this working really well. I did look at myTAP too, since we have a few MySQL instances.