Add unique index on configuration for batched_background_migrations

Also remove existing redundant index, the new index can be used the same
way.

See
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/62634#note_591636460.

Changelog: other

db/migrate/20210609013512_add_unique_index_for_batched_background_migrations.rb

+# frozen_string_literal: true
+
+class AddUniqueIndexForBatchedBackgroundMigrations < ActiveRecord::Migration[6.1]
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  TABLE_NAME = :batched_background_migrations
+  INDEX_NAME = 'index_batched_background_migrations_on_unique_configuration'
+  REDUNDANT_INDEX_NAME = 'index_batched_migrations_on_job_table_and_column_name'
+
+  def up
+    add_concurrent_index TABLE_NAME,
+      %i[job_class_name table_name column_name job_arguments],
+      unique: true,
+      name: INDEX_NAME
+
+    remove_concurrent_index_by_name TABLE_NAME, REDUNDANT_INDEX_NAME
+  end
+
+  def down
+    add_concurrent_index TABLE_NAME,
+      %i[job_class_name table_name column_name],
+      name: REDUNDANT_INDEX_NAME
+
+    remove_concurrent_index_by_name TABLE_NAME, INDEX_NAME
+  end
+end

db/schema_migrations/20210609013512

+ddd40d4fb063b3a728f4b9a214d6033f70ee3719ac769957f5295a0c5f62a5c0
\ No newline at end of file

db/structure.sql

@@ -22611,12 +22611,12 @@ CREATE INDEX index_badges_on_group_id ON badges USING btree (group_id);
 
 CREATE INDEX index_badges_on_project_id ON badges USING btree (project_id);
 
+CREATE UNIQUE INDEX index_batched_background_migrations_on_unique_configuration ON batched_background_migrations USING btree (job_class_name, table_name, column_name, job_arguments);
+
 CREATE INDEX index_batched_jobs_by_batched_migration_id_and_id ON batched_background_migration_jobs USING btree (batched_background_migration_id, id);
 
 CREATE INDEX index_batched_jobs_on_batched_migration_id_and_status ON batched_background_migration_jobs USING btree (batched_background_migration_id, status);
 
-CREATE INDEX index_batched_migrations_on_job_table_and_column_name ON batched_background_migrations USING btree (job_class_name, table_name, column_name);
-
 CREATE INDEX index_board_assignees_on_assignee_id ON board_assignees USING btree (assignee_id);
 
 CREATE UNIQUE INDEX index_board_assignees_on_board_id_and_assignee_id ON board_assignees USING btree (board_id, assignee_id);

lib/gitlab/database/background_migration/batched_migration.rb

@@ -14,6 +14,10 @@ module Gitlab
           class_name: 'Gitlab::Database::BackgroundMigration::BatchedJob',
           foreign_key: :batched_background_migration_id
 
+        validates :job_arguments, uniqueness: {
+          scope: [:job_class_name, :table_name, :column_name]
+        }
+
         scope :queue_order, -> { order(id: :asc) }
         scope :queued, -> { where(status: [:active, :paused]) }
         scope :for_configuration, ->(job_class_name, table_name, column_name, job_arguments) do

spec/factories/gitlab/database/background_migration/batched_migrations.rb

@@ -9,6 +9,7 @@ FactoryBot.define do
     job_class_name { 'CopyColumnUsingBackgroundMigrationJob' }
     table_name { :events }
     column_name { :id }
+    sequence(:job_arguments) { |n| [["column_#{n}"], ["column_#{n}_convert_to_bigint"]] }
     total_tuple_count { 10_000 }
     pause_ms { 100 }
   end

spec/lib/gitlab/database/background_migration/batched_migration_spec.rb

@@ -19,6 +19,12 @@ RSpec.describe Gitlab::Database::BackgroundMigration::BatchedMigration, type: :m
     end
   end
 
+  describe 'validations' do
+    subject { build(:batched_background_migration) }
+
+    it { is_expected.to validate_uniqueness_of(:job_arguments).scoped_to(:job_class_name, :table_name, :column_name) }
+  end
+
   describe '.queue_order' do
     let!(:migration1) { create(:batched_background_migration) }
     let!(:migration2) { create(:batched_background_migration) }