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MDL-42882 upgrade: improved SQL query

Kudos to Tim Hunt who came up with this, on mysql with 4 milion
records (and a fast SSD) the performance difference is:

Original: 36.83 sec
New query: 8.63 sec
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danpoltawski committed Feb 3, 2014
1 parent 119cf17 commit 40c761e01afa8ab3dd1df54dd3093529db81c698
Showing with 7 additions and 10 deletions.
  1. +7 −10 lib/upgradelib.php
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@@ -2097,16 +2097,13 @@ function upgrade_fix_missing_root_folders() {
global $DB, $USER;
$transaction = $DB->start_delegated_transaction();
$sql = "SELECT distinct f1.contextid, f1.component, f1.filearea, f1.itemid
FROM {files} f1 left JOIN {files} f2
ON f1.contextid = f2.contextid
AND f1.component = f2.component
AND f1.filearea = f2.filearea
AND f1.itemid = f2.itemid
AND f2.filename = '.'
AND f2.filepath = '/'
WHERE (f1.component <> 'user' or f1.filearea <> 'draft')
and f2.id is null";
$sql = "SELECT contextid, component, filearea, itemid
FROM {files}
WHERE (component <> 'user' OR filearea <> 'draft')
GROUP BY contextid, component, filearea, itemid
HAVING MAX(CASE WHEN filename = '.' AND filepath = '/' THEN 1 ELSE 0 END) = 0";
$rs = $DB->get_recordset_sql($sql);
$defaults = array('filepath' => '/',
'filename' => '.',

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