ORDER in CTE lost after GROUP BY - sql

Question

I have the following SQL
WITH tally AS (
SELECT results.answer,
results.poll_id,
count(1) AS votes
FROM (
SELECT pr.poll_id,
unnest(pr.response) AS answer
FROM poll_responses pr
LEFT JOIN polls p ON pr.poll_id = p.id
LEFT JOIN poll_collections pc ON pc.id = p.poll_collection_id
WHERE pc.id = ${pollCollectionId}
) AS results
GROUP BY results.answer, results.poll_id
),
all_choices AS (SELECT unnest(pls.choices) AS choice,
pls.id AS poll_id
FROM poll_collections pcol
INNER JOIN polls pls
ON pcol.id = pls.poll_collection_id
WHERE pcol.id = ${pollCollectionId}),
unvoted_tally AS (SELECT ac.choice AS answer,
ac.poll_id,
0 AS total
FROM all_choices ac
LEFT JOIN tally t ON t.answer = ac.choice
WHERE t.answer IS NULL),
final_tally AS (SELECT *
FROM tally
UNION
ALL
SELECT *
FROM unvoted_tally),
sorted_tally AS (
SELECT ft.*
FROM final_tally ft
ORDER BY array_position(array(SELECT choice FROM all_choices), ft.answer)
)
SELECT json_agg(poll_results.polls) AS polls
FROM (
SELECT json_array_elements(json_agg(results)) -> 'poll' AS polls
FROM (
SELECT json_build_object(
'id', st.poll_id,
'question', pls.question,
'choice-type', pls.choice_type,
'results',
json_agg(json_build_object('choice', st.answer, 'votes', st.votes)),
'chosen', pr.response
) AS poll
FROM sorted_tally st
LEFT JOIN polls pls
ON
pls.id = st.poll_id
LEFT JOIN poll_responses pr
ON
pr.poll_id = st.poll_id AND
pr.email = ${email}
GROUP BY st.poll_id, pls.choice_type, pr.response, pls.question
) AS results)
AS poll_results;
I have a poll_responses table which store the user responses of a poll. I want to order the responses in exactly the same order they are stored in the polls table - as an array e.g., {Yes, No, Maybe}.
I applied the ORDER BY array_position(array(SELECT choice FROM all_choices), ft.answer) in the sorted_tally CTE.
However, in the file SELECT after applying GROUP BY the order is lost.
Is there a way to preserve the order of the choices?
Also, are there any optimizations applicable?
Much appreciated!

Answer 1

In json_build_object or json_agg you can set ORDER BY clause. First, have the last CTE SELECT needed order expression as a new column, then run in outermost query:
CTE
...
sorted_tally AS (
SELECT ft.votes
, ft.poll_id
, ft.answer
, array_position(array(SELECT choice FROM all_choices),
ft.answer) AS choice_order
FROM final_tally ft
ORDER BY
)
Outermost Query
...
json_build_object(
'id', st.poll_id,
'question', pls.question,
'choice-type', pls.choice_type,
'results', json_agg(json_build_object('choice', st.answer,
'votes', st.votes)
ORDER BY st.choice_order),
'chosen', pr.response
) AS poll

Answer 2

ORDER BY in a CTE doesn't really matter. It may work, but SQL Server is free to re-order the rows unless you specify ORDER BY in the outermost query to order all the results.