id,summary,reporter,owner,description,type,status,priority,milestone,component,version,severity,resolution,keywords,cc
16910,Lazy evaluation for WP_Query,scribu,,"Suppose we have a 'city' post type, which is associated with a 'country' post type, such that each 'city' post has a 'country' parent post. 

Then the 'country' CPT has a 'language' taxonomy associated to it.

Now, let's say we want to find all the cities which speak a certain language.

Let's assume we already have the `'post_parent__in'` query var: #13927

We could construct a two-step query, like this:

1) Get all the countries with that language:

{{{
$country_ids = get_posts( array(
  'fields' => 'ids',
  'post_type' => 'country',
  'language' => 'french',
  'nopaging' => true
) );
}}}

2) Get all the cities belonging to that country:

{{{
$cities = get_posts( array(
  'post_type' => 'city',
  'post_parent__in' => $country_ids
) );
}}}

No custom SQL queries; fantastic!

But, if you have many many countries (not a good example, I know), you waste a lot of time passing the IDs back and forth from PHP to SQL.

Final query:

{{{
SELECT *
FROM wp_posts
WHERE post_type = 'city'
AND post_parent IN (1, 2, 3, ...)
}}}

It would be a lot more scalable to put the first query into the second, directly, as a subquery.

So, it would now look like this:

1) Get all the countries with that language:

{{{
$country_ids = get_posts( array(
  'fields' => 'ids',
  'post_type' => 'country',
  'language' => 'french',
  'nopaging' => true,

  'lazy' => true'
) );
}}}

2) Get all the cities belonging to that country:

{{{
$cities = get_posts( array(
  'post_type' => 'city',
  'post_parent__in' => $country_ids
) );
}}}

$country_ids would now be a WP_Lazy_Query object, which would just contain the subquery SQL. It would be appended to the second query, when needed:

Final query:

{{{
SELECT *
FROM wp_posts
WHERE post_type = 'city'
AND post_parent IN (
  SELECT ID
  FROM wp_posts
  WHERE post_type = 'country'
  INNER JOIN wp_terms ...
)
}}}",feature request,new,normal,Future Release,Query,,normal,,,