[wp-trac] [WordPress Trac] #35025: Performance regression in comments_template in 4.4

[WordPress Trac]

#35025: Performance regression in comments_template in 4.4
-------------------------------+--------------------------
 Reporter:  rogerhub           |       Owner:
     Type:  defect (bug)       |      Status:  new
 Priority:  normal             |   Milestone:  4.4.1
Component:  Comments           |     Version:  4.4
 Severity:  normal             |  Resolution:
 Keywords:  reporter-feedback  |     Focuses:  performance
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Changes (by boonebgorges):

 * keywords:   => reporter-feedback


Comment:

 Hi @rogerhub - Thanks for the report, and welcome to Trac!

 When the system works the way it was designed in 4.4,
 `$comment->get_children()` should result in no extra database queries,
 when called on comments that were fetched using `WP_Comment_Query` with
 `'hierarchical' => 'threaded'`. Here's the rundown:

 1. `comment_template()` calls `WP_Comment_Query` with `'hierarchical' =>
 'threaded'`. https://core.trac.wordpress.org/browser/tags/4.4/src/wp-
 includes/comment-template.php?marks=1288,1327#L1283
 2. `WP_Comment_Query::get_comments()` then calls the `fill_descendants()`
 method https://core.trac.wordpress.org/browser/tags/4.4/src/wp-includes
 /class-wp-comment-query.php?marks=443-445#L440
 3. `fill_descendants()` does only a single DB query for each hierarchy
 level across all located comments. In your case, this should mean just two
 queries: one for the top-level comments (ie, all of them), and then one
 for children of those comments (should return none, at which point db
 queries should stop)
 4. When `fill_descendants()` then builds the comment tree, it marks each
 comment with the `populated_children` flag.
 https://core.trac.wordpress.org/browser/tags/4.4/src/wp-includes/class-wp-
 comment-query.php?marks=925-928#L906
 5. When `populated_children` is set, the `get_children()` method of
 `WP_Comment` should *not* do a database query:
 https://core.trac.wordpress.org/browser/tags/4.4/src/wp-includes/class-wp-
 comment.php?marks=290-296#L279 See [34730].

 (phew)

 So, that's how it's supposed to work. (I'm going to attach a unit test
 that demonstrates that what I've described above is working, at least in
 the simple case.) If it's not working for you, it must be short-circuiting
 at one of these steps. With the above sketch in mind, do you think you
 could do some more debugging to see at which point the process is failing?

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Ticket URL: <https://core.trac.wordpress.org/ticket/35025#comment:2>
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