Add index for selecting resource_group from ci_builds

Changelog: performance

db/post_migrate/20210915202900_prepare_index_resource_group_status_commit_id_for_ci_builds.rb

+# frozen_string_literal: true
+
+class PrepareIndexResourceGroupStatusCommitIdForCiBuilds < Gitlab::Database::Migration[1.0]
+  INDEX_NAME = 'index_ci_builds_on_resource_group_and_status_and_commit_id'
+
+  def up
+    prepare_async_index :ci_builds, [:resource_group_id, :status, :commit_id],
+      where: 'resource_group_id IS NOT NULL',
+      name: INDEX_NAME
+  end
+
+  def down
+    unprepare_async_index_by_name :ci_builds, INDEX_NAME
+  end
+end

db/schema_migrations/20210915202900

+cc8f1bea8c722dfa06fc23a3dbaaacd7f1b7c5f7b529ebfab34b7c7c38e5db79
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