# In category.rb
 
def sub_questions_in_order
 questions.order('order_number')
end

(
 SELECT id, content, order_number, type, category_id FROM questions
 WHERE questions.survey_id = 2 AND questions.category_id IS NULL
)
UNION
(
 SELECT id, content, order_number, type, category_id FROM categories
 WHERE categories.survey_id = 2 AND categories.category_id IS NULL
)

WITH RECURSIVE first_level_elements AS (
 -- Non-recursive term
 (
  (
   SELECT id, content, order_number, category_id FROM questions
   WHERE questions.survey_id = 2 AND questions.category_id IS NULL
  UNION
   SELECT id, content, order_number, category_id FROM categories
   WHERE categories.survey_id = 2 AND categories.category_id IS NULL
  )
 )
 UNION
 -- Recursive Term
 SELECT q.id, q.content, q.order_number, q.category_id
 FROM first_level_elements fle, questions q
 WHERE q.survey_id = 2 AND q.category_id = fle.id
)
SELECT * from first_level_elements;

WITH RECURSIVE first_level_elements AS (
 (
  (
   SELECT id, content, order_number, category_id FROM questions
   WHERE questions.survey_id = 2 AND questions.category_id IS NULL
  UNION
   SELECT id, content, order_number, category_id FROM categories
   WHERE categories.survey_id = 2 AND categories.category_id IS NULL
  )
 )
 UNION
 (
   SELECT e.id, e.content, e.order_number, e.category_id
   FROM
   (
    -- Fetch questions AND categories
    SELECT id, content, order_number, category_id FROM questions WHERE survey_id = 2
    UNION
    SELECT id, content, order_number, category_id FROM categories WHERE survey_id = 2
   ) e, first_level_elements fle
   WHERE e.category_id = fle.id
 )
)
SELECT * from first_level_elements;

WITH RECURSIVE first_level_elements AS (
 (
  (
   SELECT id, content, category_id, array[id] AS path FROM questions
   WHERE questions.survey_id = 2 AND questions.category_id IS NULL
  UNION
   SELECT id, content, category_id, array[id] AS path FROM categories
   WHERE categories.survey_id = 2 AND categories.category_id IS NULL
  )
 )
 UNION
 (
   SELECT e.id, e.content, e.category_id, (fle.path || e.id)
   FROM
   (
    SELECT id, content, category_id, order_number FROM questions WHERE survey_id = 2
    UNION
    SELECT id, content, category_id, order_number FROM categories WHERE survey_id = 2
   ) e, first_level_elements fle
   WHERE e.category_id = fle.id
 )
)
SELECT * from first_level_elements ORDER BY path;

WHERE e.category_id = fle.id

WITH RECURSIVE first_level_elements AS (
 (
  (
   SELECT id, content, category_id, 'questions' as type, array[id] AS path FROM questions
   WHERE questions.survey_id = 2 AND questions.category_id IS NULL
  UNION
   SELECT id, content, category_id, 'categories' as type, array[id] AS path FROM categories
   WHERE categories.survey_id = 2 AND categories.category_id IS NULL
  )
 )
 UNION
 (
   SELECT e.id, e.content, e.category_id, e.type, (fle.path || e.id)
   FROM
   (
    SELECT id, content, category_id, 'questions' as type, order_number FROM questions WHERE survey_id = 2
    UNION
    SELECT id, content, category_id, 'categories' as type, order_number FROM categories WHERE survey_id = 2
   ) e, first_level_elements fle
   -- Look for children only if the type is 'categories'
   WHERE e.category_id = fle.id AND fle.type = 'categories'
 )
)
SELECT * from first_level_elements ORDER BY path;

survey = Survey.find(9)
10.times do
 category = FactoryGirl.create(:category, :survey => survey)
 6.times do
  category = FactoryGirl.create(:category, :category => category, :survey => survey)
 end
 FactoryGirl.create(:single_line_question, :category_id => category.id, :survey_id => survey.id)
end

pry(main)> Benchmark.ms { 5.times { Survey.find(9).sub_questions_using_recursive_queries }}
=> 36.839999999999996
 
pry(main)> Benchmark.ms { 5.times { Survey.find(9).sub_questions_in_order } }
=> 1145.1309999999999