This function makes posts in the admin filterable by the author

functions.php

/**
 * This function makes posts in the admin filterable by the author.
*
* https://www.nuno-sarmento.com
*/
 function ns_filter_by_the_author() {
 	$params = array(
 		'name' => 'author', // this is the "name" attribute for filter <select>
 		'show_option_all' => 'All authors' // label for all authors (display posts without filter)
 	);

 	if ( isset($_GET['user']) )
 		$params['selected'] = $_GET['user']; // choose selected user by $_GET variable

 	wp_dropdown_users( $params ); // print the ready author list
 }

 add_action('restrict_manage_posts', 'ns_filter_by_the_author');