better performance for exists() method of Query class

yiisoft#290

src/Query.php

@@ -495,7 +495,24 @@ public function exists($db = null)
         if (!empty($this->emulateExecution)) {
             return false;
         }
+
+        #better performance
+        #save last options
+        $tmpOrderBy = $this->orderBy;
+        $tmpLimit = $this->limit;
+        $tmpOffset = $this->offset;
+        $tmpSelect = $this->select;
+        $this->orderBy = [];
+        $this->limit = 1;
+        $this->offset = null;
+        $this->select = ['_id'];
         $cursor = $this->buildCursor($db);
+        #return last options
+        $this->orderBy = $tmpOrderBy;
+        $this->limit = $tmpLimit;
+        $this->offset = $tmpOffset;
+        $this->select = $tmpSelect;
+
         foreach ($cursor as $row) {
             return true;
         }