--- ./include/graphs.inc.php.orig	2017-04-19 21:30:37.000000000 +0200
+++ ./include/graphs.inc.php	2017-04-19 21:35:34.000000000 +0200
@@ -259,13 +259,10 @@
 				$sqlFrom = 'history';
 		}
 
-		foreach ($itemIds as $itemId) {
-			$sqlUnions[] = 'SELECT MIN(ht.clock) AS c FROM '.$sqlFrom.' ht WHERE ht.itemid='.zbx_dbstr($itemId);
-		}
-
 		$dbMin = DBfetch(DBselect(
-			'SELECT MIN(ht.c) AS min_clock'.
-			' FROM ('.implode(' UNION ALL ', $sqlUnions).') ht'
+			'SELECT MIN(clock) AS min_clock'.
+			' FROM '.$sqlFrom.
+			' WHERE itemid in ('.implode(',',$itemIds).')'
 		));
 
 		$min = $min ? min($min, $dbMin['min_clock']) : $dbMin['min_clock'];