add index to fix order slow issue

app/services/management/school_data_contrast_service.rb

@@ -24,10 +24,8 @@ class Management::SchoolDataContrastService
       reports = reports.where("school_name LIKE :keyword OR school_id LIKE :keyword", keyword: "%#{keyword}%")
     end
 
-    reports = reports.group(:school_id)
+    count = reports.count('distinct(school_id)')
-    count = reports.count.count
+    reports = reports.group(:school_id).limit(PAGE_SIZE).offset(offset)
-
-    reports = reports.limit(PAGE_SIZE).offset(offset)
 
     [count, reports]
   end
@@ -56,7 +54,7 @@ class Management::SchoolDataContrastService
   end
 
   def select_columns
-    "school_id, school_name,"\
+    "school_id, school_name, @total:=0, @other_total:=0,"\
     "(@total:=SUM(IF(date BETWEEN '#{format_date(params[:begin_date])}' AND '#{format_date(params[:end_date])}', #{contrast_column}, 0))) total,"\
     "(@other_total:=SUM(IF(date BETWEEN '#{format_date(params[:other_begin_date])}' AND '#{format_date(params[:other_end_date])}', #{contrast_column}, 0))) other_total,"\
     "(IF(@other_total - @total = 0, 0.0, round((@other_total - @total) / IF(@total = 0, 1, @total), 5))) percentage,"\

db/migrate/20190427025209_add_index_to_school_daily_reports.rb

@@ -0,0 +1,9 @@
+class AddIndexToSchoolDailyReports < ActiveRecord::Migration
+  def change
+    add_index :school_daily_reports, [:date, :teacher_increase_count]
+    add_index :school_daily_reports, [:date, :student_increase_count]
+    add_index :school_daily_reports, [:date, :course_increase_count]
+    add_index :school_daily_reports, [:date, :shixun_increase_count]
+    add_index :school_daily_reports, [:date, :active_user_count]
+  end
+end