Optimizes the ci_pipelines counter query

Adds an index for batch counting in usage data

changelogs/unreleased/208885-optimize-ci_pipeline-counters-related-to-the-ci-pipeline.yml

+---
+title: Optimize ci_pipelines counters in usage data
+merge_request: 26774
+author:
+type: performance

db/migrate/20200306170321_add_index_on_user_id_and_created_at_to_ci_pipelines.rb

+# frozen_string_literal: true
+
+class AddIndexOnUserIdAndCreatedAtToCiPipelines < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :ci_pipelines, [:user_id, :created_at]
+    remove_concurrent_index :ci_pipelines, [:user_id]
+  end
+
+  def down
+    add_concurrent_index :ci_pipelines, [:user_id]
+    remove_concurrent_index :ci_pipelines, [:user_id, :created_at]
+  end
+end

db/schema.rb

@@ -873,7 +873,7 @@ ActiveRecord::Schema.define(version: 2020_03_06_170531) do
     t.index ["project_id", "status", "config_source"], name: "index_ci_pipelines_on_project_id_and_status_and_config_source"
     t.index ["project_id", "status", "updated_at"], name: "index_ci_pipelines_on_project_id_and_status_and_updated_at"
     t.index ["status"], name: "index_ci_pipelines_on_status"
-    t.index ["user_id"], name: "index_ci_pipelines_on_user_id"
+    t.index ["user_id", "created_at"], name: "index_ci_pipelines_on_user_id_and_created_at"
   end
 
   create_table "ci_pipelines_config", primary_key: "pipeline_id", force: :cascade do |t|