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tags MDL-24355 Improved computation of tag correlations. Thanks Jonat…
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…han Robson for the 1.9 patch.
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Sam Hemelryk committed Feb 3, 2011
1 parent eef3578 commit 78defd5
Showing 1 changed file with 79 additions and 44 deletions.
123 changes: 79 additions & 44 deletions tag/lib.php
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Original file line number Diff line number Diff line change
Expand Up @@ -813,59 +813,94 @@ function tag_cleanup() {
* It works as a cache for a potentially heavy load query done at the 'tag_instance' table.
* So, the 'tag_correlation' table stores redundant information derived from the 'tag_instance' table.
*
* @param number $min_correlation cutoff percentage (optional, default is 2)
* @global moodle_database $DB
* @param int $mincorrelation Only tags with more than $mincorrelation correlations will
* be identified.
* @return void
*/
function tag_compute_correlations($min_correlation=2) {
function tag_compute_correlations($mincorrelation = 2) {
global $DB;

if (!$all_tags = $DB->get_records('tag')) {
return;
// This mighty one line query fetches a row from the database for every
// individual tag correlation. We then need to process the rows collecting
// the correlations for each tag id.
// The fields used by this query are as follows:
// tagid : This is the tag id, there should be at least $mincorrelation
// rows for each tag id.
// correlation : This is the tag id that correlates to the above tagid field.
// correlationid : This is the id of the row in the tag_correlation table that
// relates to the tagid field and will be NULL if there are no
// existing correlations
$sql = 'SELECT ta.tagid, tb.tagid AS correlation, co.id AS correlationid, co.correlatedtags
FROM {tag_instance} ta
LEFT JOIN {tag_instance} tb
ON (ta.itemtype = tb.itemtype AND ta.itemid = tb.itemid AND ta.tagid <> tb.tagid)
LEFT JOIN {tag_correlation} co
ON co.tagid = ta.tagid
WHERE tb.tagid IS NOT NULL
GROUP BY ta.tagid, tb.tagid
HAVING COUNT(*) > :mincorrelation
ORDER BY ta.tagid ASC, COUNT(*) DESC, tb.tagid ASC';
$rs = $DB->get_recordset_sql($sql, array('mincorrelation' => $mincorrelation));

// Set up an empty tag correlation object
$tagcorrelation = new stdClass;
$tagcorrelation->id = null;
$tagcorrelation->tagid = null;
$tagcorrelation->correlatedtags = array();

// Iterate each row of the result set and build them into tag correlations.
foreach ($rs as $row) {
if ($row->tagid != $tagcorrelation->tagid) {
// The tag id has changed so its now time to process the tag
// correlation information we have.
tag_process_computed_correlation($tagcorrelation);
// Now we reset the tag correlation object so we can reuse it and set it
// up for the current record.
$tagcorrelation = new stdClass;
$tagcorrelation->id = $row->correlationid;
$tagcorrelation->tagid = $row->tagid;
$tagcorrelation->correlatedtags = array();
}
$tagcorrelation->correlatedtags[] = $row->correlation;
}
// Update the current correlation after the last record.
tag_process_computed_correlation($tagcorrelation);

$tag_correlation_obj = new stdClass();
foreach($all_tags as $tag) {

// query that counts how many times any tag appears together in items
// with the tag passed as argument ($tag_id)
$query = "SELECT tb.tagid
FROM {tag_instance} ta JOIN {tag_instance} tb ON ta.itemid = tb.itemid
WHERE ta.tagid = ? AND tb.tagid <> ?
GROUP BY tb.tagid
HAVING COUNT(*) > ?
ORDER BY COUNT(*) DESC";
$params = array($tag->id, $tag->id, $min_correlation);

$correlated = array();

// Correlated tags happen when they appear together in more occasions
// than $min_correlation.
if ($tag_correlations = $DB->get_records_sql($query, $params)) {
foreach($tag_correlations as $correlation) {
// commented out - now done in query. kept here in case it breaks on some db
// if($correlation->nr >= $min_correlation){
$correlated[] = $correlation->tagid;
// }
}
}
// Close the recordset
$rs->close();
}

if (empty($correlated)) {
continue;
}
/**
* This function processes a tag correlation and makes changes in the database
* as required.
*
* The tag correlation object needs have both a tagid property and a correlatedtags
* property that is an array.
*
* @param stdClass $tagcorrelation
* @return bool True if the function completed, false if something was wrong.
*/
function tag_process_computed_correlation(stdClass $tagcorrelation) {
global $DB;

$correlated = implode(',', $correlated);
//var_dump($correlated);
// You must provide a tagid and correlatedtags must be set and be an array
if (empty($tagcorrelation->tagid) || !isset($tagcorrelation->correlatedtags) || !is_array($tagcorrelation->correlatedtags)) {
return false;
}

//saves correlation info in the caching table
if ($tag_correlation_obj = $DB->get_record('tag_correlation', array('tagid'=>$tag->id), 'id')) {
$tag_correlation_obj->correlatedtags = $correlated;
$DB->update_record('tag_correlation', $tag_correlation_obj);
} else {
$tag_correlation_obj = new stdClass();
$tag_correlation_obj->tagid = $tag->id;
$tag_correlation_obj->correlatedtags = $correlated;
$DB->insert_record('tag_correlation', $tag_correlation_obj);
}
// The row tagid doesn't match the current tag id which means we are onto
// the next tag. Before we switch over we need to either insert or update
// the correlation.
$tagcorrelation->correlatedtags = join(',', $tagcorrelation->correlatedtags);
if (!empty($tagcorrelation->id)) {
// The tag correlation already exists so update it
$DB->update_record('tag_correlation', $tagcorrelation);
} else {
// This is a new correlation to insert
$DB->insert_record('tag_correlation', $tagcorrelation);
}
return true;
}

/**
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