Dump big rows using CI return error memory limit

PHP Code:

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Migration extends CI_Controller {
    
    function test_ci()
    {
        ini_set('memory_limit', '2048M'); //i even tried -1 (my RAM is 8Gb)
        $this->db->save_queries = FALSE; //just make sure save_queries disabled
        $q = $this->db->query("SELECT * FROM mytable"); 
        echo $q->num_rows(); 
    }
    
    function test_native(){
        $serverName = "****";
        $connectionInfo = array( "Database"=>"****", "UID"=>"****", "PWD"=>"****" );
        $conn = sqlsrv_connect( $serverName, $connectionInfo);
        $sql = "SELECT * FROM mytable";
        $stmt = sqlsrv_query( $conn, $sql , [], ["Scrollable" => SQLSRV_CURSOR_KEYSET] );
        echo sqlsrv_num_rows( $stmt );
    }
    
} 


my database.php

PHP Code:

$active_group = 'default';
$query_builder = TRUE;

$db['default'] = array(
    'dsn'    => '',
    'hostname' => '***',
    'username' => '***',
    'password' => '***',
    'database' => '***',
    'dbdriver' => 'sqlsrv',
    'dbprefix' => '',
    'pconnect' => FALSE,
    'db_debug' => (ENVIRONMENT !== 'production'),
    'cache_on' => FALSE,
    'cachedir' => '',
    'char_set' => 'utf8',
    'dbcollat' => 'utf8_general_ci',
    'swap_pre' => '',
    'encrypt' => FALSE,
    'compress' => FALSE,
    'stricton' => FALSE,
    'failover' => array(),
    'save_queries' => FALSE //disabled save_queries
); 


i want to dump all my rows (180k++ rows) for several purpose (like migration, etc). But i always get memory_limit error. 

So i create 2 method to compared my problem:

test_ci = fatal error memory_limit blabla
test_native = 180k++

NOTE:

1. in this demo i just try to count rows to simplify the demo code. But in the real case i have have complex script to parse the result.
2. i can hack this using Limit clause. But i just want to know how to do this using CI.

i tried to googling this for an hour but still not get any luck.
did i miss something?