I have a requirement where I need to show all posts having thumbnail first and ordered by title and then remaining posts ordered by title Below is the code I tried

$the_page = (get_query_var('paged')) ? get_query_var('paged') : -1;
    $args = array(
        'post_type' => "listings",
        'post_status' => 'publish',
        //'meta_key' => '_thumbnail_id',
        'order' => 'DESC',
        'posts_per_page' => 88,
        'paged' => $the_page,

        'orderby' => 'meta_key title',
    'meta_query' => array(  //query post based on meta key
        array(
            'relation' => 'OR', // add condition if meta key is exists or not
            array(
                'key' => '_thumbnail_id', 
                'compare' => 'NOT EXISTS' // include post without _thumbnail_id key
            ),
            array(
                'key' => '_thumbnail_id', 
                'compare' => 'NOT NULL' // include post without _thumbnail_id key
            )
        )
    )

If we look closely , I have set orderby='meta_key title' and meta_key is not one of supported arguments in WP_Query Documnentation hence its being removed by WP_Query while generating sql.

In generated sql only ORDER BY wp_posts.post_title DESC is generated.

My question is: 1. do we have any other way to get posts with thumbnail first in the list ? 2. do we have a way to order result by field that is not listed in WP_Query supported arguments list ?

Any help or hint is appreciated :)

Thanks in advance.