Details
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Type:
Improvement
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Status:
Closed
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Priority:
Minor
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Resolution: Fixed
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Affects Version/s: 1.8.4, 1.9
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Fix Version/s: 1.9
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Component/s: Backup, Performance
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Labels:None
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Environment:MySQL 5.0.22
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Database:MySQL
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Affected Branches:MOODLE_18_STABLE, MOODLE_19_STABLE
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Fixed Branches:MOODLE_19_STABLE
Description
If scheduled course backups are enabled, cron.php calls schedule_backup_cron() whenever it runs. Line 36 of backup/backup_scheduled.php is
$numofrec = count_records_select ("backup_log","time > $timeafter");
This runs a full scan on mdl_backup_log, which can be a very large table depending on your site's log-retention policy. For example, we have cron.php scheduled to run every 5 minutes, and ~65,000 rows are read, simply to find out whether a backup task is currently running. The query takes around 4 seconds to complete. A workaround is to add an index to the table.
ALTER TABLE mdl_backup_log ADD INDEX (time);
This avoids the full table scan, and the query now completes in less than one hundredth of a second at our site. The drawback is that mdl_backup_log.MYI is now almost as big as mdl_backup_log.MYD I can't see any way of working around this issue in the PHP code.
Issue Links
| This issue is a clone of: | ||||
| MDL-20018 | CLONE - Scheduled course backup runs full table scan on mdl_backup_log - possible regression. |
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Activity
Hi Chris,
that exact index was added to Moodle 1.9 some time ago (it will be created as part of the 1.8 => 1.9 upgrade). Anyway, it's safe if you've created it manually in you 1.8 site: upgrade will simply skip its creation.
Closing, thanks for report, ciao 
Speed of query updated