When should you use WP_Query vs query_posts() vs get_posts()?

Question

It seems like half the tutorials in the codex and around the blogosphere use query_posts() and half use WP_Query. What's the deal?

Answer 1

• query_posts() is overly simplistic and problematic way to modify main query of a page by replacing it with new instance of the query. It is inefficient (re-runs SQL queries) and will outright fail in some circumstances (especially often when dealing with posts pagination). Any modern WP code should use more reliable methods, like making use ofpre_get_posts hook, for this purpose. TL;DR don't use query_posts() ever;

• get_posts() is very similar in usage and accepts same arguments (with some nuances, like different defaults), but returns array of posts, doesn't modify global variables and is safe to use anywhere;

• WP_Query class powers both behind the scenes, but you can also create and work with own object of it. Bit more complex, less restrictions, also safe to use anywhere.

Answer 2

The basic difference is that query_posts() is really only for modifying the current Loop. Once you're done it's necessary to reset the loop and send it on its merry way. This method is also a little easier to understand, simply because your "query" is basically a URL string that you pass to the function, like so:

query_posts('meta_key=color&meta_value=blue'); 

On the other hand, wp_query is more of a general purpose tool, and is more like directly writing MySQL queries than query_posts() is. You can also use it anywhere (not just in the Loop) and it doesn't interfere with any currently running post queries.

I tend to use wp_query more often, as it happens. Really, it's going to come down to your specific case.

Answer 3

If I recall reading right, essentially "the loop" is doing WP_Queries in the core files, but in an easier to understand way.