Add partial index on projects id when in legacy storage

This is intended to help hashed storage migration on big instances.

changelogs/unreleased/194066-add-index-to-help-hashed-storage-migration-on-big-instances.yml

+---
+title: Add Index to help Hashed Storage migration on big instances
+merge_request: 22391
+author:
+type: performance

db/migrate/20200102170221_add_storage_version_index_to_projects.rb

+# frozen_string_literal: true
+
+class AddStorageVersionIndexToProjects < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :projects, :id, where: 'storage_version < 2 or storage_version IS NULL', name: 'index_on_id_partial_with_legacy_storage'
+  end
+
+  def down
+    remove_concurrent_index :projects, :id, where: 'storage_version < 2 or storage_version IS NULL', name: 'index_on_id_partial_with_legacy_storage'
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_12_29_140154) do
+ActiveRecord::Schema.define(version: 2020_01_02_170221) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -3333,6 +3333,7 @@ ActiveRecord::Schema.define(version: 2019_12_29_140154) do
     t.index ["creator_id"], name: "index_projects_on_creator_id"
     t.index ["description"], name: "index_projects_on_description_trigram", opclass: :gin_trgm_ops, using: :gin
     t.index ["id", "repository_storage", "last_repository_updated_at"], name: "idx_projects_on_repository_storage_last_repository_updated_at"
+    t.index ["id"], name: "index_on_id_partial_with_legacy_storage", where: "((storage_version < 2) OR (storage_version IS NULL))"
     t.index ["id"], name: "index_projects_on_id_partial_for_visibility", unique: true, where: "(visibility_level = ANY (ARRAY[10, 20]))"
     t.index ["id"], name: "index_projects_on_mirror_and_mirror_trigger_builds_both_true", where: "((mirror IS TRUE) AND (mirror_trigger_builds IS TRUE))"
     t.index ["last_activity_at"], name: "index_projects_on_last_activity_at"