What is the difference between `posts_per_page` set to -1 in WP_Query and using `nopaging` parameter?

Question

WordPress Fundamental — Hard

What is the difference between `posts_per_page` set to -1 in WP_Query and using `nopaging` parameter?

Accepted Answer

Setting `posts_per_page` to -1 in WP_Query retrieves all posts but still runs SQL_CALC_FOUND_ROWS if 'no_found_rows' is not set. On the other hand, using `nopaging` parameter also retrieves all posts but may have slightly different internal handling. Both can lead to memory exhaustion on large datasets, so it's important to include 'no_found_rows' => true to avoid the expensive count query.