Create partial index for gitlab-monitor CI metrics

This creates a partial index intended to speed up queries on
`ci_builds`. Particularly, `gitlab-monitor` has rather heavy queries.
Those have been changed to only look back 7 days and benefit from this
index tremendously.

Relates to
https://gitlab.com/gitlab-org/gitlab-exporter/merge_requests/101.

changelogs/unreleased/ab-add-index-for-ci-builds-metrics.yml

+---
+title: Create partial index for gitlab-monitor CI metrics
+merge_request: 32546
+author:
+type: performance

db/migrate/20190902152329_add_index_for_ci_builds_metrics.rb

+# frozen_string_literal: true
+
+class AddIndexForCiBuildsMetrics < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'ci_builds_gitlab_monitor_metrics'
+
+  def up
+    add_concurrent_index(:ci_builds, [:status, :created_at, :project_id], where: "type = 'Ci::Build'", name: INDEX_NAME)
+  end
+
+  def down
+    remove_concurrent_index_by_name(:ci_builds, INDEX_NAME)
+  end
+end

db/schema.rb

@@ -605,6 +605,7 @@ ActiveRecord::Schema.define(version: 2019_09_02_160015) do
     t.index ["scheduled_at"], name: "partial_index_ci_builds_on_scheduled_at_with_scheduled_jobs", where: "((scheduled_at IS NOT NULL) AND ((type)::text = 'Ci::Build'::text) AND ((status)::text = 'scheduled'::text))"
     t.index ["stage_id", "stage_idx"], name: "tmp_build_stage_position_index", where: "(stage_idx IS NOT NULL)"
     t.index ["stage_id"], name: "index_ci_builds_on_stage_id"
+    t.index ["status", "created_at", "project_id"], name: "ci_builds_gitlab_monitor_metrics", where: "((type)::text = 'Ci::Build'::text)"
     t.index ["status", "type", "runner_id"], name: "index_ci_builds_on_status_and_type_and_runner_id"
     t.index ["token"], name: "index_ci_builds_on_token", unique: true
     t.index ["token_encrypted"], name: "index_ci_builds_on_token_encrypted", unique: true, where: "(token_encrypted IS NOT NULL)"