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MDL-37431 question preview: cron performance problem.

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This query was performing badly. The solution seems to be to add a
redundant JOIN and WHERE to the subquery.

I renamed the table aliases in the subquery to avoid name collisions
elsewhere. I am afraid that makes the diff rather unhelpful. The
    AND subq_qu.component = :qubacomponent2
line is new. The other lines have just had subq_ added.
This commit is contained in:
Tim Hunt
2013-01-11 13:03:28 +00:00
parent dbee98b2e6
commit 3daceb9d5c
1 changed files with 8 additions and 6 deletions
Download Patch File Download Diff File Expand all files Collapse all files
question/previewlib.php
+8 -6
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@@ -341,14 +341,16 @@ function question_preview_cron() {
'quba.component = :qubacomponent
AND NOT EXISTS (
SELECT 1
FROM {question_attempts} qa
JOIN {question_attempt_steps} qas ON qas.questionattemptid = qa.id
WHERE qa.questionusageid = quba.id
AND (qa.timemodified > :qamodifiedcutoff
OR qas.timecreated > :stepcreatedcutoff)
FROM {question_attempts} subq_qa
JOIN {question_attempt_steps} subq_qas ON subq_qas.questionattemptid = subq_qa.id
JOIN {question_usages} subq_qu ON subq_qu.id = subq_qa.questionusageid
WHERE subq_qa.questionusageid = quba.id
AND subq_qu.component = :qubacomponent2
AND (subq_qa.timemodified > :qamodifiedcutoff
OR subq_qas.timecreated > :stepcreatedcutoff)
)
',
array('qubacomponent' => 'core_question_preview',
array('qubacomponent' => 'core_question_preview', 'qubacomponent2' => 'core_question_preview',
'qamodifiedcutoff' => $lastmodifiedcutoff, 'stepcreatedcutoff' => $lastmodifiedcutoff));
question_engine::delete_questions_usage_by_activities($oldpreviews);