(Bug1364703) Add caching to ArtefactType::get_plugin_name

Add caching of results to ArtefactType::get_plugin_name.
In a test export of 1000 users via admin/users/bulkexport.php,
this patch reduced the number of calls to get_field from 78,000
to 26 and reduced the execution time from 115 seconds to 92 seconds.

Change-Id: Ie7b2e39089bed1c3cd4844748b21f1ffb251f8dd
Signed-off-by: Nigel Cunningham <nigelc@catalyst-au.net>

htdocs/artefact/lib.php

@@ -438,7 +438,14 @@ abstract class ArtefactType implements IArtefactType {
     }
 
     public function get_plugin_name() {
-        return get_field('artefact_installed_type', 'plugin', 'name', $this->get('artefacttype'));
+        static $cache = array();
+
+        $type = $this->get('artefacttype');
+        if (!isset($cache[$type])) {
+            $cache[$type] = get_field('artefact_installed_type', 'plugin', 'name', $type);
+        }
+
+        return $cache[$type];
     }
 
     /**