Wordpress and wp_ajax

I want to use $WPDB class outside Wordpress context in an external Ajax handling script.

For this in general is needed to load the core Worpdress files.

After loading those core files I can make calls to the MySQL tables using the $wpdb class, but loading the core Wordpress files is a lot of overhead just to run a couple of queries. I was looking for a better solution and I find 'wp_ajax'
and 'wp_ajax_nopriv' hooks, but on Wordpress site and on Google I couldn't find any good documentation or tutorial.

I need to transfer, using ajax, back and forth data as Json.
Could anyone provide me with more information or a good example about this?
Thx

 

Maybe this will help you:

$path  = '';

if(!defined('WP_LOAD_PATH')){
	$root = dirname(dirname(dirname(dirname(dirname(__FILE__))))).'/';

	if(file_exists($root.'wp-load.php')){
        define('WP_LOAD_PATH',$root);
	}else{
        if(file_exists($path.'wp-load.php')){
            define('WP_LOAD_PATH',$path);
        }else{
            exit("Cannot find wp-load.php");
        }
	}
}

require_once(WP_LOAD_PATH.'wp-load.php');

global $wpdb;

Code (markup):

 

The script is working, but is using the same method that you presented, loading the core Wordpress files, but is a lot of overhead so I founded an wordpress hook wp_ajax to use instead, but is not documented.So I'm searching for a complete demo or tutorial.

In wordpress functions.php file I have a function which control a form, which can have tens to hundreds of fields.This function is used to modify different fields in all the site(pages and posts). It is a mass editing function.On submit on the form, with jquery $post I send Json data to a script, that send me back json data, as array.But instead using jquery directly I want to use the wp_ajax hook, so I can have access to $wpdb.