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