218 lines
8.0 KiB
Markdown
218 lines
8.0 KiB
Markdown
---
|
|
stage: Enablement
|
|
group: Database
|
|
info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
|
|
---
|
|
|
|
# `NOT NULL` constraints
|
|
|
|
> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/38358) 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`:
|
|
|
|
```ruby
|
|
class CreateDbGuides < ActiveRecord::Migration[6.0]
|
|
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`:
|
|
|
|
```ruby
|
|
class AddExtendedTitleToSprints < ActiveRecord::Migration[6.0]
|
|
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 requires multiple steps split into at least two
|
|
different releases:
|
|
|
|
1. 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](not_null_constraints.md#not-null-constraints-on-large-tables) section for more information.
|
|
|
|
- Create an issue for the next milestone to validate the `NOT NULL` constraint.
|
|
|
|
1. 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:
|
|
|
|
```ruby
|
|
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 will not be affected and you'll
|
|
still be 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`:
|
|
|
|
```ruby
|
|
class AddNotNullConstraintToEpicsDescription < ActiveRecord::Migration[6.0]
|
|
include Gitlab::Database::MigrationHelpers
|
|
|
|
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 will help 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, please contact the Database team for advice.
|
|
|
|
Back to our example, the epics table is not considerably large nor frequently accessed,
|
|
so we are going to add a post-deployment migration for the 13.0 milestone (current),
|
|
`db/post_migrate/20200501000002_cleanup_epics_with_null_description.rb`:
|
|
|
|
```ruby
|
|
class CleanupEpicsWithNullDescription < ActiveRecord::Migration[6.0]
|
|
include Gitlab::Database::MigrationHelpers
|
|
|
|
# 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 will scan 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`:
|
|
|
|
```ruby
|
|
class ValidateNotNullConstraintOnEpicsDescription < ActiveRecord::Migration[6.0]
|
|
include Gitlab::Database::MigrationHelpers
|
|
|
|
disable_ddl_transaction!
|
|
|
|
def up
|
|
validate_not_null_constraint :epics, :description
|
|
end
|
|
|
|
def down
|
|
# no-op
|
|
end
|
|
end
|
|
```
|
|
|
|
## `NOT NULL` constraints on large tables
|
|
|
|
If you have to clean up a nullable column for a [high-traffic table](../migration_style_guide.md#high-traffic-tables)
|
|
(for example, the `artifacts` in `ci_builds`), your background migration will go on for a while and
|
|
it will need an additional [background migration cleaning up](../background_migrations.md#cleaning-up)
|
|
in the release after adding the data migration.
|
|
|
|
In that rare case you will need 3 releases end-to-end:
|
|
|
|
1. Release `N.M` - Add the `NOT NULL` constraint and the background-migration to fix the existing records.
|
|
1. Release `N.M+1` - Cleanup the background migration.
|
|
1. Release `N.M+2` - Validate the `NOT NULL` constraint.
|
|
|
|
For these cases, please consult the database team early in the update cycle. The `NOT NULL`
|
|
constraint may not be required or other options could exist that do not affect really large
|
|
or frequently accessed tables.
|