`NOT NULL` constraints

Introduced in GitLab 13.0.

All attributes that should not have NULL as a value, should be defined as NOT NULL columns in the database.

Depending on the application logic, NOT NULL columns should either have a presence: true validation defined in their Model or have a default value as part of their database definition. As an example, the latter can be true for boolean attributes that should always have a non-NULL value, but have a well defined default value that the application does not need to enforce each time (for example, active=true).

Create a new table with `NOT NULL` columns

When adding a new table, all NOT NULL columns should be defined as such directly inside create_table.

For example, consider a migration that creates a table with two NOT NULL columns, db/migrate/20200401000001_create_db_guides.rb:

class CreateDbGuides < Gitlab::Database::Migration[1.0]
  def change
    create_table :db_guides do |t|
      t.bigint :stars, default: 0, null: false
      t.bigint :guide, null: false
    end
  end
end

Add a `NOT NULL` column to an existing table

With PostgreSQL 11 being the minimum version in GitLab 13.0 and later, adding columns with NULL and/or default values has become much easier and the standard add_column helper should be used in all cases.

For example, consider a migration that adds a new NOT NULL column active to table db_guides, db/migrate/20200501000001_add_active_to_db_guides.rb:

class AddExtendedTitleToSprints < Gitlab::Database::Migration[1.0]
  def change
    add_column :db_guides, :active, :boolean, default: true, null: false
  end
end

Add a `NOT NULL` constraint to an existing column

Adding NOT NULL to existing database columns usually requires multiple steps split into at least two different releases. If your table is small enough that you don't need to use a background migration, you can include all these in the same merge request. We recommend to use separate migrations to reduce transaction durations.

The steps required are:

Release N.M (current release)
- Ensure the constraint is enforced at the application level (that is, add a model validation).
- Add a post-deployment migration to add the NOT NULL constraint with validate: false.
- Add a post-deployment migration to fix the existing records.
  
  NOTE: Depending on the size of the table, a background migration for cleanup could be required in the next release. See the NOT NULL constraints on large tables section for more information.
- Create an issue for the next milestone to validate the NOT NULL constraint.
Release N.M+1 (next release)
- Validate the NOT NULL constraint using a post-deployment migration.

Example

Considering a given release milestone, such as 13.0, a model validation has been added into epic.rb to require a description:

class Epic < ApplicationRecord
  validates :description, presence: true
end

The same constraint should be added at the database level for consistency purposes. We only want to enforce the NOT NULL constraint without setting a default, as we have decided that all epics should have a user-generated description.

After checking our production database, we know that there are epics with NULL descriptions, so we can not add and validate the constraint in one step.

NOTE: Even if we did not have any epic with a NULL description, another instance of GitLab could have such records, so we would follow the same process either way.

Prevent new invalid records (current release)

We first add the NOT NULL constraint with a NOT VALID parameter, which enforces consistency when new records are inserted or current records are updated.

In the example above, the existing epics with a NULL description are not affected and you are still able to update records in the epics table. However, when you try to update or insert an epic without providing a description, the constraint causes a database error.

Adding or removing a NOT NULL clause requires that any application changes are deployed first. Thus, adding a NOT NULL constraint to an existing column should happen in a post-deployment migration.

Still in our example, for the 13.0 milestone example (current), we add the NOT NULL constraint with validate: false in a post-deployment migration, db/post_migrate/20200501000001_add_not_null_constraint_to_epics_description.rb:

class AddNotNullConstraintToEpicsDescription < Gitlab::Database::Migration[1.0]
  disable_ddl_transaction!

  def up
    # This will add the `NOT NULL` constraint WITHOUT validating it
    add_not_null_constraint :epics, :description, validate: false
  end

  def down
    # Down is required as `add_not_null_constraint` is not reversible
    remove_not_null_constraint :epics, :description
  end
end

Data migration to fix existing records (current release)

The approach here depends on the data volume and the cleanup strategy. The number of records that must be fixed on GitLab.com is a nice indicator that helps us decide whether to use a post-deployment migration or a background data migration:

If the data volume is less than 1000 records, then the data migration can be executed within the post-migration.
If the data volume is higher than 1000 records, it's advised to create a background migration.

When unsure about which option to use, contact the Database team for advice.

Back to our example, the epics table is not considerably large nor frequently accessed, so we add a post-deployment migration for the 13.0 milestone (current), db/post_migrate/20200501000002_cleanup_epics_with_null_description.rb:

class CleanupEpicsWithNullDescription < Gitlab::Database::Migration[1.0]
  # With BATCH_SIZE=1000 and epics.count=29500 on GitLab.com
  # - 30 iterations will be run
  # - each requires on average ~150ms
  # Expected total run time: ~5 seconds
  BATCH_SIZE = 1000

  disable_ddl_transaction!

  class Epic < ActiveRecord::Base
    include EachBatch

    self.table_name = 'epics'
  end

  def up
    Epic.each_batch(of: BATCH_SIZE) do |relation|
      relation.
        where('description IS NULL').
        update_all(description: 'No description')
    end
  end

  def down
    # no-op : can't go back to `NULL` without first dropping the `NOT NULL` constraint
  end
end

Validate the `NOT NULL` constraint (next release)

Validating the NOT NULL constraint scans the whole table and make sure that each record is correct.

Still in our example, for the 13.1 milestone (next), we run the validate_not_null_constraint migration helper in a final post-deployment migration, db/post_migrate/20200601000001_validate_not_null_constraint_on_epics_description.rb:

class ValidateNotNullConstraintOnEpicsDescription < Gitlab::Database::Migration[1.0]
  disable_ddl_transaction!

  def up
    validate_not_null_constraint :epics, :description
  end

  def down
    # no-op
  end
end