WP_Query eliminate memory overflow

Whenever WP_Query() runs on big blogs with many posts, it takes lot of memory, so we get memory overflow.

$args = array(
     'post_status'     => 'publish',
     'post_type'       => array( 'post' ),
     'posts_per_page'  => -1
);
      $query = null;
      wp_reset_query();
      $query = new WP_Query($args);

Why there is Memory Overflow?

WP_Query actually loads every matching post into memory, including the full post contents; and how smart WordPress is; it may also load meta data for every post as pre-fetch action. This easily leads to the memory overflow.

But there is a million dollars trick, where you pass 'fields' => 'ids' into WP_Query to simply return a list of IDs.

$args = array(
     'post_status'     => 'publish',
     'post_type'       => array( 'post' ),
     'posts_per_page'  => -1,
     'fields' => 'ids'
);
      $query = null;
      wp_reset_query();
      $query = new WP_Query($args);

Now, how is that for a memory trick.