Pull latitude / longitude from a custom table

<?php

add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'YOUR_FACET_NAME_HERE' == $params['facet_name'] ) {
        global $wpdb;

        $sql = $wpdb->prepare( "SELECT lat, lng FROM {$wpdb->prefix}cp_ad_geocodes WHERE post_id = %d LIMIT 1", $params['post_id'] );
        $result = $wpdb->get_row( $sql );

        if ( null !== $result ) {
            $params['facet_value'] = $result->lat;
            $params['facet_display_value'] = $result->lng;
        }
        else {
            return false;
        }
    }
    return $params;
}, 10, 2 );