An update to a talk I gave at WordCamp Portland 2011, "You Don't Know Query" is an advanced development talk from March 25, 2012, in Utrecht, Netherlands.

1. WordCamp Netherlands 2012

2. Andrew Nacin
Core Developer of WordPress
and Tech Ninja at Audrey Capital
@nacin on Twitter
nacin@wordpress.org

3. You Don’t Know Query

4. What do you know?

5. Conditional Tags
is_author( ), is_home( ), etc.

6. query_posts( )

7. Ways to query
query_posts( )
new WP_Query( )
get_posts( )

8. The Loop
while ( have_posts( ) ) :
the_post( );
endwhile;

9. A secondary loop
$query = new WP_Query( … );
while ( $query->have_posts( ) ) :
$query->the_post( );
endwhile;

10. An array of posts
$result = get_posts( … );
foreach ( $result as $post_obj ) {
}

11. What don’t you know?

12. Every query object has its
own methods
is_author( ) is the same as calling
$wp_query->is_author( )

13. function is_author( ) {
global $wp_query;
return $wp_query->is_author( );
}

14. With the regular loop
while ( have_posts( ) ) :
the_post( );
if ( is_author( ) )
echo "An author query.";
endwhile;

15. With the regular loop
while ( have_posts( ) ) :
the_post( );
if ( $wp_query->is_author( ) )
echo "An author query.";
endwhile;

16. A secondary loop
$query = new WP_Query( … );
while ( $query->have_posts( ) ) :
$query->the_post( );
if ( $query->is_author( ) )
echo "An author query.";
endwhile;

17. A secondary loop
$query = new WP_Query( … );
while ( $query->have_posts( ) ) :
$query->the_post( );
if ( $query->is_author( ) )
echo "An author query.";
endwhile;

18. A secondary loop
$query = new WP_Query( … );
while ( $query->have_posts( ) ) :
$query->the_post( );
if ( $query->is_author( ) )
echo "An author query.";
endwhile;

19. If you do:
$my_query = new WP_Query( $query );
You can do:
while ( $my_query->have_posts( ) ) :
$my_query->the_post( );
endwhile;
wp_reset_postdata( );

20. Why do we call functions like
wp_reset_postdata( ) and
wp_reset_query( )?
What about using query_posts( )?
How can you alter a query? How
can you alter the main query?

21. What is the main query,
and why should I care?

22. wp-blog-header.php
// Load the WordPress bootstrap
require './wp-load.php';
// Do magic
wp( );
// Decide which template files to load
require WPINC . '/template-loader.php';

23. Let's look in the bootstrap:
$wp_the_query = new WP_Query();
$wp_query =& $wp_the_query;

24. Quick lesson on PHP references
$a = 4;
$b =& $a;
$b = 2;
var_dump( $a ); // int(2)
$a = 6;
var_dump( $b ); // int(6)

25. So:
So the real main query is in
$wp_the_query.
And a live copy of it is stored in
$wp_query.

26. wp-blog-header.php
// Load the WordPress bootstrap
require './wp-load.php';
// Do magic
wp( );
// Decide which template files to load
require WPINC . '/template-loader.php';

27. wp-blog-header.php
// Load the WordPress bootstrap
require './wp-load.php';
// Do magic
wp( );
// Decide which template files to load
require WPINC . '/template-loader.php';

28. What is that wp( ) call?
function wp( $query_vars = '' ) {
global $wp;
$wp->main( $query_vars );
}

29. Holy $!@?, what just
happened?

30. In the bootstrap:
$wp = new WP( );
So there's a wp( ) function,
and a WP class.

31. class WP {
. . .
function main( ) {
$this->init( );
$this->parse_request( );
$this->send_headers( );
$this->query_posts( );
$this->handle_404( );
$this->register_globals( );
. . .

32. class WP {
. . .
function main( ) {
$this->init( );
$this->parse_request( );
$this->send_headers( );
$this->query_posts( );
$this->handle_404( );
$this->register_globals( );
. . .

33. WP::parse_request( )
— Parses the URL using WP_Rewrite
— Sets up query variables for WP_Query
WP::query_posts( ) {
global $wp_the_query;
$wp_the_query->query( $this->query_vars );
}

34. Boom.
SELECT SQL_CALC_FOUND_ROWS
wp_posts.*
FROM wp_posts
WHERE 1=1
AND wp_posts.post_type = 'post'
AND wp_posts.post_status = 'publish'
ORDER BY wp_posts.post_date DESC
LIMIT 0, 10

35. wp-blog-header.php
// Load WordPress.
require './wp-load.php';
// Parse what to query. Then query it.
wp( );
// Load the theme.
require WPINC . '/template-loader.php';

36. Before we get to the theme,
we have your posts.
Got it?

37. Then why do we do this?
query_posts( 'author=-5' );
get_header( );
while( have_posts( ) ) :
the_post( );
endwhile;
get_footer( );

38. That's running 2* queries!
One, the query WordPress
thought we wanted.
Two, this new one you're
actually going to use.

39. * Actually, WP_Query
doesn't run just one query.
It usually runs four.

40. 1. Get me my posts:
SELECT
SQL_CALC_FOUND_ROWS …
FROM wp_posts LIMIT 0, 10
2. How many posts exist?
SELECT FOUND_ROWS( )
3. Get all metadata for these posts.
4. Get all terms for these posts.

41. (You can turn these off selectively…)
$my_query = new WP_Query( array(
'no_found_rows' => true,
'update_post_meta_cache' => false,
'update_post_term_cache' => false,
) );

42. </aside>

43. PROTIP
‘Measure twice, cut once’
is bad for performance.

44. Other problems with
query_posts( )

45. Pagination breaks.
WordPress calculated
paging using the query it
did, not the query you did.

46. query_posts( array(
'author' => -5,
'posts_per_page' => 25,
) );
This will not work well.

47. You easily mess up globals.
This can break widgets and
more.

48. query_posts( ) is bad.
Do we agree?

49. Introducing pre_get_posts
class WP_Query {
. . .
function &get_posts() {
$this->parse_query();
// Huzzah!
do_action_ref_array( 'pre_get_posts',
array( &$this ) );
. . .

50. A truly awesome hook.
function nacin_alter_home( $query ) {
if ( $query->is_home( ) )
$query->set( 'author', '-5' );
}
add_action( 'pre_get_posts', 'nacin_alter_home' );

51. Still with us?
Good, ‘cause here’s where
things get complicated.

52. 'pre_get_posts' fires for every post
query:
— get_posts( )
— new WP_Query( )
— That random recent posts widget your client
installed without you knowing.
— Everything.

53. What if I just want it on the
main query?

54. $wp_the_query makes a
triumphant return.

55. Main query only!
function nacin_alter_home( $query ) {
global $wp_the_query;
if ( $wp_the_query === $query
&& $query->is_home() )
$query->set( 'author', '-5' );
}
add_action( 'pre_get_posts', 'nacin_alter_home' );

56. Hmm. How does this work?
$wp_the_query should never be modified. It
holds the main query, forever.
$wp_query keeps a live reference to
$wp_the_query, unless you use
query_posts( ).

57. query_posts( 'author=-5' );
while ( have_posts( ) ) :
the_post( );
endwhile;
wp_reset_query( );

58. query_posts( 'author=-5' );
while ( have_posts( ) ) :
the_post( );
endwhile;
wp_reset_query( );

59. function query_posts( $query ) {
// Break the reference to $wp_the_query
unset( $wp_query );
$wp_query =& new WP_Query( $query );
return $wp_query;
}

60. query_posts( 'author=-5' );
while ( have_posts( ) ) :
the_post( );
endwhile;
wp_reset_query( );

61. function wp_reset_query( ) {
// Restore reference to $wp_the_query
unset( $wp_query );
$wp_query =& $wp_the_query;
// Reset the globals, too.
wp_reset_postdata( );
}

62. Calling the_post( )?
wp_reset_query( ) will reset $wp_query
and the globals.
Calling $my_query->the_post( )?
wp_reset_postdata( ) will reset the globals.

63. New in WordPress 3.3!
Rather than:
$wp_the_query === $other_query_object
You can call:
$other_query_object->is_main_query( )
is_main_query( ), the function, will act on
$wp_query, like any other conditional tag.

64. What about page
templates?

65. /* Template: My Template */
query_posts( $query_string .
'&author=-5&posts_per_page=25' );
get_header( );
while ( have_posts( ) ) :
the_post( );
endwhile;

66. function nacin_my_template( $query ) {
if ( ! $query->is_main_query( ) )
return;
if ( ! is_page_template( 'my-template.php' ) )
return;
$query->set( 'author', '-5' );
$query->set( 'posts_per_page', '25' );
}
add_action( 'pre_get_posts',
'nacin_my_template' );

67. Some Lessons
Every WP_Query object has methods that
mimic the global conditional tags.
The global conditional tags apply to
$wp_query, the main or current query.
$wp_query is always the main query, unless
you use query_posts( ). Restore it with
wp_reset_query( ).

68. And Finally
pre_get_posts is a powerful and flexible hook.
Just use it properly.
Always check if you're modifying the main
query using $query->is_main_query( )

69. Thanks! Questions?
@nacin