You Don't Know Query (WordCamp Netherlands 2012)

Andrew Nacin
Core Developer of WordPress
and Tech Ninja at Audrey Capital

@nacin on Twitter
nacin@wordpress.org

Conditional Tags

is_author( ), is_home( ), etc.

Ways to query

query_posts( )
new WP_Query( )
get_posts( )

The Loop

while ( have_posts( ) ) :
the_post( );
endwhile;

A secondary loop

$query = new WP_Query( … );
while ( $query->have_posts( ) ) :
$query->the_post( );
endwhile;

An array of posts

$result = get_posts( … );
foreach ( $result as $post_obj ) {

}

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

function is_author( ) {
global $wp_query;

return $wp_query->is_author( );
}

With the regular loop

the_post( );
if ( is_author( ) )
echo "An author query.";
endwhile;

With the regular loop

the_post( );
if ( $wp_query->is_author( ) )
endwhile;

A secondary loop

$query = new WP_Query( … );
while ( $query->have_posts( ) ) :
$query->the_post( );
if ( $query->is_author( ) )
endwhile;

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( );

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?

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

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';

Let's look in the bootstrap:

$wp_the_query = new WP_Query();
$wp_query =& $wp_the_query;

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

So:
So the real main query is in
$wp_the_query.

And a live copy of it is stored in
$wp_query.

What is that wp( ) call?

function wp( $query_vars = '' ) {
global $wp;

$wp->main( $query_vars );
}

Holy $!@?, what just
happened?

In the bootstrap:

$wp = new WP( );

So there's a wp( ) function,
and a WP class.

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

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 );
}

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

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';

Before we get to the theme,
we have your posts.

Got it?

Then why do we do this?

query_posts( 'author=-5' );
get_header( );
while( have_posts( ) ) :
the_post( );
endwhile;
get_footer( );

That's running 2* queries!

One, the query WordPress
thought we wanted.

Two, this new one you're
actually going to use.

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

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.

(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,
) );

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

You Don't Know Query (WordCamp Netherlands 2012)

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