Describe the reasoning behind the modified query

Author: dtdesign

wcfsetup/install/files/lib/system/session/SessionHandler.class.php

@@ -588,9 +588,17 @@ private function getExistingSession(string $sessionID): bool
 
                 $spiderIdentifier = SpiderHandler::getInstance()->getIdentifier(UserUtil::getUserAgent());
                 if ($spiderIdentifier === null) {
-                    $condition->add('(sessionID = ? AND spiderIdentifier IS NULL)', [
-                        $row['sessionID'],
-                    ]);
+                    // MySQL 8.4 does not like `OR spiderIdentifier = ?` with the
+                    // placeholder being supplied with `NULL`. Instead of raising
+                    // a warning or something, it will cause the query planer to
+                    // pick some odd indices to work with instead of focusing on
+                    // the session id.
+                    //
+                    // Technically we don’t care for the spider identifier at
+                    // this point which means that we can optimize the query
+                    // right away. This query also lives in the hot path of a
+                    // request which makes it even more reasonable.
+                    $condition->add('sessionID = ?', [$row['sessionID']]);
                 } else {
                     $condition->add('(sessionID = ? OR spiderIdentifier = ?)', [
                         $row['sessionID'],