Add index on section_name_id on ci_build_trace_sections table

This was causing user deletions to time out.

Closes #44008

changelogs/unreleased/sh-add-section-name-index.yml

+---
+title: Add index on section_name_id on ci_build_trace_sections table
+merge_request:
+author:
+type: performance

db/migrate/20180308052825_add_section_name_id_index_on_ci_build_trace_sections.rb

+class AddSectionNameIdIndexOnCiBuildTraceSections < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    # MySQL may already have this as a foreign key
+    unless index_exists?(:ci_build_trace_sections, :section_name_id)
+      add_concurrent_index :ci_build_trace_sections, :section_name_id
+    end
+  end
+
+  def down
+    # We cannot remove index for MySQL because it's needed for foreign key
+    if Gitlab::Database.postgresql?
+      remove_concurrent_index :ci_build_trace_sections, :section_name_id
+    end
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180307012445) do
+ActiveRecord::Schema.define(version: 20180308052825) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -264,6 +264,7 @@ ActiveRecord::Schema.define(version: 20180307012445) do
 
   add_index "ci_build_trace_sections", ["build_id", "section_name_id"], name: "index_ci_build_trace_sections_on_build_id_and_section_name_id", unique: true, using: :btree
   add_index "ci_build_trace_sections", ["project_id"], name: "index_ci_build_trace_sections_on_project_id", using: :btree
+  add_index "ci_build_trace_sections", ["section_name_id"], name: "index_ci_build_trace_sections_on_section_name_id", using: :btree
 
   create_table "ci_builds", force: :cascade do |t|
     t.string "status"