php - Close session after executing database query -

i have little experience joomla , sql , appreciate help! using joomla 2.5 , querying data database , storing in memory following code:

function getlist() {     $mainframe = jfactory::getapplication('site');         $db = jfactory::getdbo();         $query = "  select                      *                                       #__listuser                                      $db->setquery( $query );"         $rows = $db->loadobjectlist();         return $rows; } 

i have 3 questions,

1. when query database, new db session opened, need close after or automatic?
   
2. do know of more efficient way achieve method (a user session memory size 11mb!)
   
3. is there security issue accessing database using method?

thank much! appreciated!

the code should (i don't see how can work now):

function getlist() {    // $mainframe = jfactory::getapplication('site'); // don't need line!         $db = jfactory::getdbo();         $query = "  select                      *                                       #__listuser                                      1=1"; // condition extract selected rows         $db->setquery( $query ); // sets query , it's joomla, not sql.         $rows = $db->loadobjectlist();         return $rows; } 

please note .... needs condition (else if want rows, remove , follows)

1. you don't need close it
2. 11mb not due query, try adding limit 0,1 (to return 1 row) you'll see memory doesn't change much. turn on debug in global configuration, , reload component. @ bottom of page you'll see extensions eating memory. 11mb acceptable though on installations.
3. should create condition using input params, make sure $db->quote() values prevent sql-injection.