Migration to add/drop primary key constraints for composite keys.

db/optional_migrations/composite_primary_keys.rb

+# This migration adds a primary key constraint to tables
+# that only have a composite unique key.
+#
+# This is not strictly relevant to Rails (v4 does not
+# support composite primary keys). However this becomes
+# useful for e.g. PostgreSQL's logical replication (pglogical)
+# which requires all tables to have a primary key constraint.
+#
+# In that sense, the migration is optional and not strictly needed.
+class CompositePrimaryKeysMigration < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  Index = Struct.new(:table, :name, :columns)
+
+  TABLES = [
+    Index.new(:issue_assignees, 'index_issue_assignees_on_issue_id_and_user_id', %i(issue_id user_id)),
+    Index.new(:user_interacted_projects, 'index_user_interacted_projects_on_project_id_and_user_id', %i(project_id user_id)),
+    Index.new(:merge_request_diff_files, 'index_merge_request_diff_files_on_mr_diff_id_and_order', %i(merge_request_diff_id relative_order)),
+    Index.new(:merge_request_diff_commits, 'index_merge_request_diff_commits_on_mr_diff_id_and_order', %i(merge_request_diff_id relative_order)),
+    Index.new(:project_authorizations, 'index_project_authorizations_on_user_id_project_id_access_level', %i(user_id project_id access_level)),
+    Index.new(:push_event_payloads, 'index_push_event_payloads_on_event_id', %i(event_id)),
+    Index.new(:schema_migrations, 'unique_schema_migrations', %(version)),
+  ]
+
+  disable_ddl_transaction!
+
+  def up
+    return unless Gitlab::Database.postgresql?
+
+    disable_statement_timeout
+    TABLES.each do |index|
+      add_primary_key(index)
+    end
+  end
+
+  def down
+    return unless Gitlab::Database.postgresql?
+
+    disable_statement_timeout
+    TABLES.each do |index|
+      remove_primary_key(index)
+    end
+  end
+
+  private
+  def add_primary_key(index)
+    execute "ALTER TABLE #{index.table} ADD PRIMARY KEY USING INDEX #{index.name}"
+  end
+
+  def remove_primary_key(index)
+    temp_index_name = "#{index.name[0..58]}_old"
+    rename_index index.table, index.name, temp_index_name if index_exists_by_name?(index.table, index.name)
+
+    # re-create unique key index
+    add_concurrent_index index.table, index.columns, unique: true, name: index.name
+
+    # This also drops the `temp_index_name` as this is owned by the constraint
+    execute "ALTER TABLE #{index.table} DROP CONSTRAINT IF EXISTS #{temp_index_name}"
+  end
+end
+

lib/tasks/migrate/composite_primary_keys.rake

+namespace :gitlab do
+  namespace :db do
+    desc 'GitLab | Adds primary keys to tables that only have composite unique keys'
+    task composite_primary_keys_add: :environment do
+      require Rails.root.join('db/optional_migrations/composite_primary_keys')
+      CompositePrimaryKeysMigration.new.up
+    end
+
+    desc 'GitLab | Removes previously added composite primary keys'
+    task composite_primary_keys_drop: :environment do
+      require Rails.root.join('db/optional_migrations/composite_primary_keys')
+      CompositePrimaryKeysMigration.new.down
+    end
+  end
+end