Altering the query such that the second left join called does not return so many results seems to get it.

If you change the query above to the one below you get the same results, in a fraction of the time (query above reduced to 2 seconds. The additional line is on the second left join having it only include attempts for the quiz we are looking at as opposed to all quizzes. ' where qa7qa.questionusageid in (select uniqueid from mdl_quiz_attempts where quiz = '434' )

SELECT
CONCAT(u.id, '#', COALESCE(quiza.attempt, 0)) AS uniqueid,
quiza.uniqueid AS usageid,
quiza.id AS attempt,
u.id AS userid,
u.idnumber,
u.firstname,
u.lastname,
u.picture,
u.imagealt,
u.institution,
u.department,
u.email,
quiza.sumgrades,
quiza.timefinish,
quiza.timestart,
CASE WHEN quiza.timefinish = 0 THEN null
WHEN quiza.timefinish > quiza.timestart THEN quiza.timefinish - quiza.timestart
ELSE 0 END AS duration, COALESCE((
SELECT MAX(qqr.regraded)
FROM mdl_quiz_overview_regrades qqr
WHERE qqr.questionusageid = quiza.uniqueid
), -1) AS regraded,
qa7.fraction * qa7.maxmark AS qsgrade7
FROM
mdl_user u
LEFT JOIN mdl_quiz_attempts quiza ON
quiza.userid = u.id AND quiza.quiz = '434'
LEFT JOIN (
SELECT
qa7qa.id AS questionattemptid,
qa7qa.questionusageid,
qa7qa.slot,
qa7qa.behaviour,
qa7qa.questionid,
qa7qa.variant,
qa7qa.maxmark,
qa7qa.minfraction,
qa7qa.flagged,
qa7qa.questionsummary,
qa7qa.rightanswer,
qa7qa.responsesummary,
qa7qa.timemodified,
qa7qas.id AS attemptstepid,
qa7qas.sequencenumber,
qa7qas.state,
qa7qas.fraction,
qa7qas.timecreated,
qa7qas.userid

FROM mdl_question_attempts qa7qa
JOIN mdl_question_attempt_steps qa7qas ON
qa7qas.id = (
SELECT MAX(id)
FROM mdl_question_attempt_steps
WHERE questionattemptid = qa7qa.id
) where qa7qa.questionusageid in (select uniqueid from mdl_quiz_attempts where quiz = '434' )
) qa7 ON qa7.questionusageid = quiza.uniqueid AND qa7.slot = '7'
WHERE quiza.id IS NOT NULL AND quiza.preview = 0
ORDER BY qsgrade7 ASC, firstname ASC, quiza.id ASC LIMIT 0, 250;

1. Create a quiz that allows multiple attempts per students, and in separate groups mode (make sure there are several groups set up in the course). Make sure the quiz has at least several questions. Preferably more.

2. Create many quiz attempts, as many different students.

(Ideally, find a real quiz from a copy of a real Moodle site.)

3. Then for many combinations of options
a. Show / download
b. Only show / download attempts
c. Page size
d. Include (for the responses report)
d. Choices in the initials bar.
e. Which group is selected (or all groups)
f. Which column the table is sorted on. Focus in particular on sorting by individual question grade, or Question/response/right answer (depending on overview, or responses report.)

4. Compare the report with and without this patch, and verify that it is identical.

5. Do this for both the overview (grades) and responses report.