improves performance of work package autocompleter

* add db hint on mysql to use primary key for the ORDER BY
* restrict the fiels to search on based on the character of the search query. e.g. if there are alphabet characters in the search query, we don't have to search on the id column

app/controllers/work_packages/auto_completes_controller.rb

@@ -29,37 +29,71 @@
 
 require 'rack/utils'
 
-class WorkPackages::AutoCompletesController < ApplicationController
+class WorkPackages::AutoCompletesController < ::ApplicationController
+  before_filter :on_no_valid_scope_404
+
   def index
+    @work_packages = work_package_with_id | work_packages_by_subject_or_id
+
+    respond_to do |format|
+      format.html do render layout: false end
+      format.any(:xml, :json) { render request.format.to_sym => wp_hashes_with_string(@work_packages) }
+    end
+  end
+
+  private
+
+  def on_no_valid_scope_404
     scope = determine_scope
     if scope.nil?
       render_404
-      return
+
+      return false
     end
+  end
 
+  def work_package_with_id
+    scope = determine_scope
     query_term = params[:q].to_s
-    @work_packages = []
-    # query for exact ID matches first, to make an exact match the first result of autocompletion
+
     if query_term =~ /\A\d+\z/
-      @work_packages |= scope.visible.where(id: query_term.to_i)
+      scope.visible.where(id: query_term.to_i)
+    else
+      []
     end
+  end
 
-    sql_query = ["LOWER(#{WorkPackage.table_name}.subject) LIKE :q OR
-                  CAST(#{WorkPackage.table_name}.id AS CHAR(13)) LIKE :q",
-                 { q: "%#{query_term.downcase}%" }]
+  def work_packages_by_subject_or_id
+    scope = determine_scope
+    query_term = params[:q].to_s
 
-    @work_packages |= scope.visible
-                      .where(sql_query)
-                      .order("#{WorkPackage.table_name}.id ASC") # :id does not work because...
-                      .limit(10)
+    if query_term =~ /\A\d+\z/
+      sql_query = ["#{WorkPackage.table_name}.subject LIKE :q OR
+                   CAST(#{WorkPackage.table_name}.id AS CHAR(13)) LIKE :q",
+                   { q: "%#{query_term}%" }]
+    else
+      sql_query = ["LOWER(#{WorkPackage.table_name}.subject) LIKE :q",
+                   { q: "%#{query_term.downcase}%" }]
+    end
 
-    respond_to do |format|
-      format.html do render layout: false end
-      format.any(:xml, :json) { render request.format.to_sym => wp_hashes_with_string(@work_packages) }
+    # The filter on subject in combination with the ORDER BY on id
+    # seems to trip MySql's usage of indexes on the order statement
+    # I haven't seen similar problems on postgresql but there might be as the
+    # data at hand was not very large.
+    #
+    # For MySql we are therefore helping the DB optimizer to use the correct index
+
+    if ActiveRecord::Base.connection_config[:adapter] == 'mysql2'
+      scope = scope.from("#{WorkPackage.table_name} USE INDEX(PRIMARY)")
     end
-  end
 
-  private
+    scope
+      .visible
+      .where(sql_query)
+      .order("#{WorkPackage.table_name}.id ASC") # :id does not work because...
+      .limit(10)
+      .includes(:type)
+  end
 
   def wp_hashes_with_string(work_packages)
     work_packages.map do |work_package|
@@ -77,16 +111,18 @@ class WorkPackages::AutoCompletesController < ApplicationController
   end
 
   def determine_scope
-    project = find_project
+    @scope ||= begin
+      project = find_project
 
-    if params[:scope] == 'relatable'
-      return nil unless project
+      if params[:scope] == 'relatable'
+        return nil unless project
 
-      Setting.cross_project_work_package_relations? ? WorkPackage.all : project.work_packages
-    elsif params[:scope] == 'all' || project.nil?
-      WorkPackage.all
-    else
-      project.work_packages
+        Setting.cross_project_work_package_relations? ? WorkPackage.all : project.work_packages
+      elsif params[:scope] == 'all' || project.nil?
+        WorkPackage.all
+      else
+        project.work_packages
+      end
     end
   end
 end