Answers for "wp_query return all posts id"

wp_query post by category id

$args=array(
    'posts_per_page' => 50, 
    'post_type' => 'my_custom_type'
    'cat' => $cat_id,
);
$wp_query = new WP_Query( $args );

wp get_posts return ids

get_posts(array(
    'fields'          => 'ids', // Only get post IDs
    'posts_per_page'  => -1
));