Optimise `wp_delete_comment` as called from `wp_delete_post`

[peterwilsoncc]

Remove the need for reparenting comments in wp_delete_comment by deleting comments in descending order.

Potentially accept an array of comments.

[Mte90]

Looking right now in the code I didn't see forcing of an order on SQL query, maybe in the meantime this issue was fixed?

[peterwilsoncc]

Replying to Mte90:

Looking right now in the code I didn't see forcing of an order on SQL query, maybe in the meantime this issue was fixed?

This is correct, no order is specified in wp_delete_posts (​see wp-includes/post.php#L2490). In most cases the DB defaults to insertion order, in this case ascending by date.

[Mte90]

So what is the purpose of the ticket, is not so much clear right now for me :-)

[johnbillion]

@peterwilsoncc Can you clarify the exact optimisation you'd like to make here? Is it the order in which wp_delete_comment() gets called or is it optimisation within the function? Thanks!

[peterwilsoncc]

@Mjte90 Please accept my apologies for not seeing your follow up question so long ago.

Step 1: Optimise call within wp_delete_post()

• sort $comment_ids from highest to lowest ​before deleting each comment in this loop.
• this allows the ​reparenting step to be skipped when deleting each comment.

Step 2: Accept array of comments when calling wp_delete_comment()

• when deleting an array of comments, calculate reparenting in bulk.
• if a comment's parent is to be deleted, reparent to the first ancestor not being deleted

The SQL queries may become something like this:

-- Comment threads
-- 1 -> 3 -> 5, 6 -> 11
-- 2 -> 7 -> 9
-- 8 -> 4 -> 10
-- 12 -> 13 -> 14

-- wp_delete_comment( [ 5, 6, 7, 8, 11, 12, 13 ] );

-- Determine if a parent comment is being deleted from a comment that is not being deleted

SELECT comment_ID, comment_parent
  FROM wp_comments 
  WHERE comment_parent IN (5, 6, 7, 8, 11, 12, 13) 
    AND comment_ID NOT IN (5, 6, 7, 8, 11, 12, 13);

-- result: (4, 8), (9, 7), (14, 13)
-- Comments 8, 7, 13 are being deleted and parents of comments 4, 9, 14.

-- Determine parents of 8, 7, 13 to determining grandparents of 4, 9, 14
SELECT comment_ID, comment_parent FROM wp_comments WHERE comment_ID IN (8, 7, 13);

-- result (8, 0), (7, 2), (13, 12)
-- No parent comment 8 thus no grandparent for comment 4. Comment 4 should be reparented to 0
-- Parent of comment 7 is 2, thus comment 7 should be reparented to 2
-- Parent of comment 13 is 12 (grandparent comment 14), 12 will be deleted.

-- Repeat above step for comment 12 to determine comment 14's closest ancestor 
SELECT comment_ID, comment_parent FROM wp_comments WHERE comment_ID IN (12);

-- result: (12, 0)
-- 12 has no parent comment, thus comment 14 should be reparented to 0

-- No further looping required to determine reparenting

-- Reparent as follows
-- Comment 4, reparent to 0
-- Comment 7, reparent to 3
-- Comment 14, reparent to 0

Step 2 is the more difficult step, as it requires fired hooks to remain unchanged.