Overhead of using Yii::app()->db multiple times? - yii

Question

if I have to do multiple queries in a row, is it better to do this:
$connection = Yii::app()->db;
once... and then keep using $connection, or is there an overhead if I have multiple functions like this:
function mainFunction() {
$dbResult1 = dbresult1();
$dbResult2 = dbresult2();
$dbResult2 = dbresult3();
}
function dbresult1() {
$connection = Yii::app()->db;
// do stuff
return $result;
}
function dbresult2() {
$connection = Yii::app()->db;
// do stuff
return $result;
}
function dbresult3() {
$connection = Yii::app()->db;
// do stuff
return $result;
}
Would it be better to do this:
function mainFunction() {
$connection = Yii::app()->db;
// do stuff with $connection for $dbResult1
// do stuff with $connection for $dbResult2
// do stuff with $connection for $dbResult3
}
?

Answer 1

You can trace the source code to see what happens when you call Yii::app()->db:
Yii::app() returns the static app property of YiiBase. You can see the source code here.
public static function app()
{
return self::$_app;
}
Yii::app()->db here it gets more interesting, because something gets looked up. You can see the source code here.
public function getDb()
{
return $this->getComponent('db');
}
getComponent() is a method of CModule you find the source code here.
public function getComponent($id,$createIfNull=true)
{
if(isset($this->_components[$id]))
return $this->_components[$id];
elseif(isset($this->_componentConfig[$id]) && $createIfNull)
{
$config=$this->_componentConfig[$id];
if(!isset($config['enabled']) || $config['enabled'])
{
Yii::trace("Loading \"$id\" application component",'system.CModule');
unset($config['enabled']);
$component=Yii::createComponent($config);
$component->init();
return $this->_components[$id]=$component;
}
}
}
As you see Yii::app()->db results in a few method calls and an array lookup. If performance is highly critical you might should cache the db instance. Else I would target on writing clean and readable code and won't care about such little tweaks.