Optimize mysql query by not using/inquiring same table twice - mysql

Question

Thanks for your kind words and wisdom in helping me optimize the following query as I would like to use the "answers" table only once. As well, if there is any explanation accompanying it, I would be delighted.
SELECT
score.user_id,
name_email.NAME,
score.question_id,
TRUNCATE(GREATEST(0, SUM(answers.is_correct / IF(answers.is_correct = 1, possible_answers.good_answers, possible_answers.wrong_answers))) ,2) AS score
FROM score
INNER JOIN (
SELECT
id,
email,
CONCAT(users.f_name, " ", users.l_name) as NAME
FROM
users
)AS name_email
INNER JOIN answers
INNER JOIN (
SELECT
answers.question_id,
count(*) AS total_answers,
SUM(IF(is_correct = 1, 1, 0)) AS good_answers,
SUM(IF(is_correct = -1, 1, 0)) AS wrong_answers
FROM answers
GROUP BY answers.question_id
) AS possible_answers
WHERE score.answer_id = answers.id
AND possible_answers.question_id = score.question_id
AND score.user_id = name_email.id
-- AND score.user_id = 2
-- AND score.question_id = 1007
GROUP BY score.question_id, score.user_id
ORDER BY score.user_id, score.question_id
;

Answer 1

Note all queries (including your original) are improved by adding a composite index on the scores table, like so:
ALTER TABLE `score` ADD INDEX `score_comp_uaq` (`user_id`, `answer_id`, `question_id`);
Your final score formula refers to answers in 2 different contexts:
the possible answers, and
the actual answers
So, one way to avoid using the answers table twice is to use an outer join so we can access both the possible and the actual in one pass. However in this approach there is a problem of knowing which questions a user has attempted so while it reduces the passes over the answers table it requires 2 passes of the scores table.
SELECT
d.user_id
, CONCAT(users.f_name, " ", users.l_name) as NAME
, d.question_id
, d.good_answers
, d.wrong_answers
, d.possible_good
, d.possible_wrong
, COALESCE((d.good_answers / NULLIF(d.possible_good,0)),0) x
, COALESCE((d.wrong_answers / NULLIF(d.possible_wrong,0)),0) y
FROM (
SELECT
u.user_id
, a.question_id
, SUM(case when s.user_id IS NOT NULL AND a.is_correct = 1 then 1 else 0 end) AS good_answers
, SUM(case when s.user_id IS NOT NULL AND a.is_correct =-1 then 1 else 0 end) AS wrong_answers
, SUM(case when a.is_correct = 1 then 1.0 else 0.0 end) AS possible_good
, SUM(case when a.is_correct =-1 then 1.0 else 0.0 end) AS possible_wrong
from answers a
INNER join (select distinct user_id, question_id from score) u
ON a.question_id = u.question_id
left join score s ON u.user_id = s.user_id
AND a.question_id = s.question_id
AND a.id = s.answer_id
GROUP BY
u.user_id
, a.question_id
) d
INNER JOIN users ON d.user_id = users.id
;
While that may be an option I believe the next query offers a better explain plan:
SELECT
s.user_id
, CONCAT(users.f_name, " ", users.l_name) as NAME
, s.question_id
, s.good_answers
, s.wrong_answers
, pa.possible_good
, pa.possible_wrong
, COALESCE((s.good_answers / NULLIF(pa.possible_good,0)),0) x
, COALESCE((s.wrong_answers / NULLIF(pa.possible_wrong,0)),0) y
FROM (
select
score.user_id
, answers.question_id
, count(*) AS total_answers
, SUM(IF(is_correct = 1, 1, 0)) AS good_answers
, SUM(IF(is_correct = -1, 1, 0)) AS wrong_answers
from score
INNER JOIN answers ON score.answer_id = answers.id
GROUP BY
score.user_id
, answers.question_id
) s
INNER JOIN (
SELECT
answers.question_id
, count(*) AS total_answers
, SUM(IF(is_correct = 1, 1.0, 0.0)) AS possible_good
, SUM(IF(is_correct = -1, 1.0, 0.0)) AS possible_wrong
FROM answers
GROUP BY answers.question_id
) AS pa ON s.question_id = pa.question_id
INNER JOIN users ON s.user_id = users.id
;
Notice that I have not attempted to reproduce your final "score" formula. I feel that you may want to amend how you do that after seeing these options, but the columns aliased "x" and "y" should allow you to arrange a final formula.
This SQLfiddle allows you to inspect both the above and your original.
first query:
+----+-------------+-------+--------+------------------------------------------------------+----------------+---------+------------------+------+----------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+--------+------------------------------------------------------+----------------+---------+------------------+------+----------+----------------------------------------------+
| 1 | PRIMARY | | ALL | | | | | 4 | 100.00 | |
| 1 | PRIMARY | users | eq_ref | PRIMARY | PRIMARY | 4 | d.user_id | 1 | 100.00 | |
| 2 | DERIVED | | ALL | | | | | 4 | 100.00 | Using where; Using temporary; Using filesort |
| 2 | DERIVED | a | ref | question_id | question_id | 4 | u.question_id | 1 | 100.00 | |
| 2 | DERIVED | s | ref | user_id,answer_choice,question_choice,score_comp_uaq | answer_choice | 5 | db_9_2522be.a.id | 1 | 100.00 | Using where |
| 3 | DERIVED | score | index | score_comp_uaq | score_comp_uaq | 14 | | 4 | 100.00 | Using index; Using temporary |
+----+-------------+-------+--------+------------------------------------------------------+----------------+---------+------------------+------+----------+----------------------------------------------+
second query:
+----+-------------+---------+--------+---------------+----------------+---------+-----------------------------+------+----------+-----------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+--------+---------------+----------------+---------+-----------------------------+------+----------+-----------------------------------------------------------+
| 1 | PRIMARY | | ALL | | | | | 4 | 100.00 | |
| 1 | PRIMARY | users | eq_ref | PRIMARY | PRIMARY | 4 | s.user_id | 1 | 100.00 | |
| 1 | PRIMARY | | ref | | | 4 | s.question_id | 2 | 100.00 | |
| 3 | DERIVED | answers | index | question_id | question_id | 4 | | 19 | 100.00 | |
| 2 | DERIVED | score | index | answer_choice | score_comp_uaq | 14 | | 4 | 100.00 | Using where; Using index; Using temporary; Using filesort |
| 2 | DERIVED | answers | eq_ref | PRIMARY | PRIMARY | 4 | db_9_2522be.score.answer_id | 1 | 100.00 | |
+----+-------------+---------+--------+---------------+----------------+---------+-----------------------------+------+----------+-----------------------------------------------------------+
original query:
+----+-------------+---------+--------+------------------------------------------------------+----------------+---------+-------------------------------+------+----------+-----------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+--------+------------------------------------------------------+----------------+---------+-------------------------------+------+----------+-----------------------------------------------------------+
| 1 | PRIMARY | score | index | user_id,answer_choice,question_choice,score_comp_uaq | score_comp_uaq | 14 | | 4 | 100.00 | Using where; Using index; Using temporary; Using filesort |
| 1 | PRIMARY | answers | eq_ref | PRIMARY | PRIMARY | 4 | db_9_2522be.score.answer_id | 1 | 100.00 | |
| 1 | PRIMARY | | ref | | | 4 | db_9_2522be.score.user_id | 2 | 100.00 | |
| 1 | PRIMARY | | ref | | | 4 | db_9_2522be.score.question_id | 2 | 100.00 | |
| 3 | DERIVED | answers | index | question_id | question_id | 4 | | 19 | 100.00 | |
| 2 | DERIVED | users | ALL | | | | | 9 | 100.00 | |
+----+-------------+---------+--------+------------------------------------------------------+----------------+---------+-------------------------------+------+----------+-----------------------------------------------------------+