Add an index for project_id in project_import_data to improve

performance

We see that many slow queries on GitLab.com are dominated by finding
the project import data for a specific project. Adding an index is
the most straightforward way of fixing this.

Closes #23748

changelogs/unreleased/add-project-import-data-index.yml

+---
+title: Add an index for project_id in project_import_data to improve performance
+merge_request: 
+author: 

db/migrate/20161106185620_add_project_import_data_project_index.rb

+class AddProjectImportDataProjectIndex < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def change
+    add_concurrent_index :project_import_data, :project_id
+  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: 20161103171205) do
+ActiveRecord::Schema.define(version: 20161106185620) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -867,6 +867,8 @@ ActiveRecord::Schema.define(version: 20161103171205) do
     t.string "encrypted_credentials_salt"
   end
 
+  add_index "project_import_data", ["project_id"], name: "index_project_import_data_on_project_id", using: :btree
+
   create_table "projects", force: :cascade do |t|
     t.string "name"
     t.string "path"