Add project_id, user_id, status, ref index to ci_pipelines

After adding username filter to pipelines, we realized this missing
index.

We exclude source=12 because we don't display child pipelines in index

changelogs/unreleased/217748-pipeline-index-endpoint-performance.yml

+---
+title: Add project_id, user_id, status, ref index to ci_pipelines
+merge_request: 33290
+author:
+type: performance

db/migrate/20200528125905_add_project_id_user_id_status_ref_index_to_ci_pipelines.rb

+# frozen_string_literal: true
+
+class AddProjectIdUserIdStatusRefIndexToCiPipelines < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  SOURCE_PARENT_PIPELINE = 12
+
+  def up
+    add_concurrent_index(
+      :ci_pipelines,
+      [:project_id, :user_id, :status, :ref],
+      where: "source != #{SOURCE_PARENT_PIPELINE}"
+    )
+  end
+
+  def down
+    remove_concurrent_index(
+      :ci_pipelines,
+      [:project_id, :user_id, :status, :ref],
+      where: "source != #{SOURCE_PARENT_PIPELINE}"
+    )
+  end
+end

db/structure.sql

@@ -9408,6 +9408,8 @@ CREATE INDEX index_ci_pipelines_on_project_id_and_status_and_config_source ON pu
 
 CREATE INDEX index_ci_pipelines_on_project_id_and_status_and_updated_at ON public.ci_pipelines USING btree (project_id, status, updated_at);
 
+CREATE INDEX index_ci_pipelines_on_project_id_and_user_id_and_status_and_ref ON public.ci_pipelines USING btree (project_id, user_id, status, ref) WHERE (source <> 12);
+
 CREATE INDEX index_ci_pipelines_on_project_idandrefandiddesc ON public.ci_pipelines USING btree (project_id, ref, id DESC);
 
 CREATE INDEX index_ci_pipelines_on_status ON public.ci_pipelines USING btree (status);
@@ -13776,6 +13778,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200527152116
 20200527152657
 20200528123703
+20200528125905
 20200603073101
 \.