Wrong Use of flush_rewrite_rules in CPT register function

Hi There,

we found that in "includes/class-betterdocs-docs-post-type.php" file plugin is registering CPT and calling flush_rewrite_rules in there. And you can see this function “register_post” is attached to init hook. Which means that flush_rewrite_rules is called on each page call hence causing too much load on database.

And we found that after commenting this function there was 50% reduction in database and web server loads.

Better way:
you can call this when plugin is activated or you can find some good examples done by other developers too.

Like this:

function myplugin_activate() {
    // register taxonomies/post types here
    flush_rewrite_rules();
}

register_activation_hook( __FILE__, 'myplugin_activate' );

function myplugin_deactivate() {
    flush_rewrite_rules();
}
register_deactivation_hook( __FILE__, 'myplugin_deactivate' );

And when plugin settings is updated by ADMIN in backend.

or

third approach suggested by Jack Johansson
https://wordpress.stackexchange.com/questions/315001/flush-rewrite-rules-on-init-or-rest-api-init
here is sample code, which if your rewrite rules are present then don’t flush

add_action( 'init', 'wpse315001_flush_rewrite_rules', 99999 );
function wpse315001_flush_rewrite_rules(){

    // Get the rewrite rules
    $rules = get_option('rewrite_rules');

    // Check if your semi-unique rule exists in this string
    if ( ! strpos( $rules, 'your-rewrite-rules' ) ) {
        // Flush'em All!
        flush_rewrite_rules();
    }
}