This document discusses how WordPress handles URL requests and database queries. It explains how URLs are parsed into query variables, how those variables are used to generate SQL queries against the database tables, and how the template system selects which files to use to display the content. Specific examples are provided for common cases like home page queries, category/tag lookups, and custom post type queries. Filters and actions are also discussed that can be used to modify and extend WordPress' default querying and display behavior.

1. © 2010 Hal Stern
Some Rights Reserved
Parsing Strange:
URL to SQL to HTML
Hal Stern
snowmanonfire.com
slideshare.net/freeholdhal
headshot by Richard Stevens
http://dieselsweeties.com

2. © 2010 Hal Stern
Some Rights Reserved
Why Do You Care?
• Database performance = user experience
• A little database expertise goes a long way
• Taxonomies for more than sidebar lists
• Custom post types
• WordPress as a powerful CMS >> blog
> Change default behaviors
> Defy the common wisdom
> Integrate other content sources/filters
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3. © 2010 Hal Stern
Some Rights Reserved
Flow of Control
• Web server URL manipulation
> Real file or permalink URL?
• URL to query variables
> What to display? Tag? Post? Category?
• Query variables to SQL generation
> How exactly to get that content?
• Template file selection
> How will content be displayed?
• Content manipulation
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4. © 2010 Hal Stern
Some Rights Reserved
Whose File Is This?
• User URL request passed to web server
• Web server checks
.htaccess file
> WP install root
> Other .htaccess
files may interfere
• Basic rewriting rules:
If file or directory URL doesn’t exist, start
WordPress via index.php
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<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /whereyouputWordPress/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

5. © 2010 Hal Stern
Some Rights Reserved
Example Meta Fail: 404 Not Found
• Access broken image URLs for
unintended results: no 404 pages!
myblog/images/not-a-pic.jpg!
• Web server can’t find file, assumes it’s a
permalink, hands to WP
• WP can’t interpret it, so defaults to home
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myblog/
myblog/wp-content (etc)
myblog/images

6. © 2010 Hal Stern
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What Happens Before The Loop
• Parse URL into a query
• Set conditionals & select templates
• Execute the query & cache results
• Run the Loop:
<?php
if (have_posts()) :
while (have_posts()) :
the_post();
//loop content
endwhile;
endif;
?>
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7. © 2010 Hal Stern
Some Rights Reserved
Examining the Query String
• SQL passed to MySQL in WP_Query
object’s request element
• Brute force: edit theme footer.php
to
see main loop’s query for displayed page
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<?php
global $wp_query;
echo ”SQL for this page ";
echo $wp_query->request;
echo "<br>";
?>

8. © 2010 Hal Stern
Some Rights Reserved
“Home Page” Query Deconstruction
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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' OR
wp_posts.post_status = 'private’)
ORDER BY wp_posts.post_date DESC LIMIT 0, 10
Get all fields from posts table, but limit number of returned rows
Only get posts, and those that are published or private to the user
Sort the results by date in descending order
Start results starting with record 0 and up to 10 more results

9. © 2010 Hal Stern
Some Rights Reserved
Query Parsing
• parse_request() method of WP_Query
extracts query variables from URL
• Execute rewrite rules
> Pick off ?p=67 style http GET variables
> Match permalink structure
> Match keywords like “author” and “tag”
> Match custom post type slugs
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10. © 2010 Hal Stern
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Query Variables to SQL
• Query type: post by title, posts by category
or tag, posts by date
• Variables for the query
> Slug values for category/tags
> Month/day numbers
> Explicit variable values
• post_type variable has been around for
a while; CPT queries fill in new values
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11. © 2010 Hal Stern
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Simple Title Slug Parsing
• Rewrite matches root of permalink,
extracts tail of URL as a title slug
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SELECT wp_posts.* FROM wp_posts WHERE 1=1 AND YEAR
(wp_posts.post_date)='2010' AND wp_posts.post_name = 'premio-
sausage' AND wp_posts.post_type = 'post' ORDER BY
wp_posts.post_date DESC
/2010/premio-sausage

12. © 2010 Hal Stern
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CPT Query Variables
• Register CPT with a custom query variable
'query_var' => 'ebay'
• Variable works in URLs like built-ins
myblog.com/?ebay=current_items
myblog.com/?ebay=cool_searches
• Variable value matches CPT title slug
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13. © 2010 Hal Stern
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WordPress Meta Data
• Common DB mechanics for all meta data
> Categories, tags, custom taxonomies
• Normalized down to 3 tables
> Terms: word strings and their slugs
> Taxonomies: collections of terms
> Relationships: terms attached to posts
• It’s so simple it gets really complex.
Really.
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14. © 2010 Hal Stern
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Graphs and JOIN Operations
• WordPress maps tags and categories 1:N
to posts (each term in many posts)
• You need to punch MySQL to handle
hierarchical, multi-valued relations
> INNER JOIN builds intermediate tables on
common key values
• Following link in a graph is equivalent to
an INNER JOIN on tables of linked items
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15. © 2010 Hal Stern
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WordPress Taxonomy Tables
• Term relationships table maps
N terms to each post
• Term taxonomy maps N
terms to each taxonomy
• Term table has slugs for URL
mapping
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wp_term_relationships
object_id
term_taxonomy_id
wp_posts
post_id
….
post_date
…
post_content
wp_term_taxonomy
term_taxonomy_id
term_id
taxonomy
description
wp_terms
term_id
name
slug

16. © 2010 Hal Stern
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SELECT SQL_CALC_FOUND_ROWS wp_posts.* FROM wp_posts
INNER JOIN wp_term_relationships ON
(wp_posts.ID = wp_term_relationships.object_id)
INNER JOIN wp_term_taxonomy ON
(wp_term_relationships.term_taxonomy_id =
wp_term_taxonomy.term_taxonomy_id)
INNER JOIN wp_terms ON
(wp_term_taxonomy.term_id = wp_terms.term_id)
WHERE 1=1 AND wp_term_taxonomy.taxonomy = 'post_tag' AND wp_terms.slug IN
('premio') AND wp_posts.post_type = 'post' AND (wp_posts.post_status =
'publish' OR wp_posts.post_status = 'private') GROUP BY wp_posts.ID ORDER
BY wp_posts.post_date DESC LIMIT 0, 10
Taxonomy Lookup
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/tag/premio

17. © 2010 Hal Stern
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More on Canonical URLs
• Canonical URLs improve SEO
• WordPress is really good about generating
301 Redirects for non-standard URLs
• Example: URL doesn’t appear to match a
permalink, WordPress does prediction
> Use “LIKE title%” in WHERE clause
> Matches “title” as initial substring with %
wildcard
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18. © 2010 Hal Stern
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Modifying the Query
• Brute force isn’t necessarily good
> Using query_posts() ignores all previous
parsing, runs a new SQL query
• Filter query_vars
> Change default parsing (convert any day to a
week’s worth of posts, for example)
• Actions parse_query & parse_request
> Access WP_Query object before execution
> is_xx() conditionals are already set
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19. © 2010 Hal Stern
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SQL Generation Filters
• posts_where
> More explicit control over query variable to
SQL grammar mapping
• posts_join
> Add or modify JOIN operations for other graph
relationships
• Many other filters
> Change grouping of results
> Change ordering of results
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20. © 2010 Hal Stern
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Custom Post Types
• Change SQL WHERE clause on post type
> wp_posts.post_type=‘ebay’
• Add new rewrite rules for URL parsing similar
to category & tag
> Set slug in CPT registration array
'rewrite' => array ("slug" => “ebay”)
• Watch out for competing, overwritten or
unflushed rewrite entries
<?php echo "<pre>”;
print_r(get_option('rewrite_rules'));
echo "</pre>”;
?>
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21. © 2010 Hal Stern
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Applications
• Stylized listings
> Category sorted alphabetically
> Use posts as listings of resources (jobs,
clients, events) – use CPT for more control
• Custom URL slugs
> Add rewrite rules to match slug and set query
variables
• Joining other social graphs
> Suggested/related content
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22. © 2010 Hal Stern
Some Rights Reserved
Template File Selection
• is_x() conditionals set in query parsing
• Used to drive template selection
> is_tag() looks for tag-slug, tag-id, then tag
> Full search hierarchy in Codex
• template_redirect action
> Called in the template loader
> Add actions to override defaults
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23. © 2010 Hal Stern
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HTML Generation
• Done in the_post() method
• Raw content retrieved from MySQL
> Short codes interpreted
> CSS applied
• Some caching plugins generate and store
HTML, so YMMV
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24. © 2010 Hal Stern
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More Complex Example
• For category or tag templates, intersperse
custom post types into the results, sorting
by post date or alphabetically by title.
• Attach filter to posts_where:
if (query is for a category or tag)
$where = ‘(‘ . $where . ‘)’ .
“OR post_type=‘extras’” ;
• This changes the database query…..
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25. © 2010 Hal Stern
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Matching Tags
• What if you want to match tags from user-
selected posts to tagged CPT posts?
• Need to get the list of tags from the main
loop query
• In template, create a new query looking for
post_type=‘cpt type’
• Use JOIN to match collected tags to tags
in the CPT (or other custom) taxonomy
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26. © 2010 Hal Stern
Some Rights Reserved
Why Do You Care?
• User experience improvement
> JOINS are expensive
> Large post table & repetitive SELECTs = slow
> Running query once keeps cache warm
> Category, permalink, title slug choices matter
• More CMS, less “blog”
> Alphabetical sort
> Adding taxonomy/social graph elements
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27. © 2010 Hal Stern
Some Rights Reserved
Resources
• Core files where SQL stuff happens
> query.php
> post.php
> canonical.php
> rewrite.php
• Template
loader
search
path
> http://codex.wordpress.org/Template_Hierarchy
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28. © 2010 Hal Stern
Some Rights Reserved
Contact (Shameless Plug)
Hal Stern
freeholdhal@gmail.com
@freeholdhal
snowmanonfire.com
facebook.com/hal.stern
slideshare.net/freeholdhal
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