Fix 500 for pipeline charts page

This MR creates a new index which will
improve the query performance and avoid
the query to timeout and return a 500.

changelogs/unreleased/mo-add-index-to-ci-pipeline.yml

+---
+title: Fix 500 for pipeline charts page
+merge_request: 38226
+author:
+type: fixed

db/migrate/20200729202222_add_index_to_ci_pipeline_project_id_created_at.rb

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

db/schema_migrations/20200729202222

+2976f459ac9cd0780e90077ebe4ce5ca8dc41e62b4dab1f96e39738624ad9d04
\ No newline at end of file

db/structure.sql

@@ -19148,6 +19148,8 @@ CREATE INDEX index_ci_pipelines_on_merge_request_id ON public.ci_pipelines USING
 
 CREATE INDEX index_ci_pipelines_on_pipeline_schedule_id ON public.ci_pipelines USING btree (pipeline_schedule_id);
 
+CREATE INDEX index_ci_pipelines_on_project_id_and_created_at ON public.ci_pipelines USING btree (project_id, created_at);
+
 CREATE INDEX index_ci_pipelines_on_project_id_and_id_desc ON public.ci_pipelines USING btree (project_id, id DESC);
 
 CREATE UNIQUE INDEX index_ci_pipelines_on_project_id_and_iid ON public.ci_pipelines USING btree (project_id, iid) WHERE (iid IS NOT NULL);