Prepare temporary index to support event.target_types backfill for project events

What does this MR do and why?

Prepare temporary index to be used for Create `BackfillEventsTargetTypeForProjects` ba... (#565788 - closed)

PoC for the complete solution: Draft: Resolve "'target_type' parameter for Eve... (!202890 - closed)

Backfill script

# frozen_string_literal: true

module Gitlab
  module BackgroundMigration
    class BackfillProjectEventsTargetType < BatchedMigrationJob
      PROJECT_ACTIONS = [1, 5, 8, 9, 11].freeze # Defined in Events::PROJECT_ACTIONS

      # rubocop:disable Database/AvoidScopeTo -- supported by an index `tmp_idx_events_missing_project_target_type`
      scope_to ->(relation) { relation.where(target_type: nil, action: PROJECT_ACTIONS).where.not(project_id: nil) }
      # rubocop:enable Database/AvoidScopeTo

      operation_name :back_project_events_target_type
      feature_category :groups_and_projects

      def perform
        each_sub_batch do |sub_batch|
          sub_batch.update_all(
            target_type: 'Project',
            target_id: Arel.sql('project_id')
          )
        end
      end
    end
  end
end

Example batching query plan: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/43148/commands/131931

References

• Closes #565787 (closed)
• Bug issue: &19098 (closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

Run bin/rails db:migrate

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