Determine page from post permalink in paginated forum - php

Question

The posts in my forum all have an individual permalink.
The posts are paginated and the users can sort them by date and rating.
When a permalink is accessed, the page the post resides on needs to be calculated due to the pagination.
The posts are stored with an auto incrementing id in a mysql innodb table.
At the moment I use the following for the calculation when the posts are sorted by their score (rating):
<?php
// These variables originate from the corresponding uri segments
// For example: http://domain.tld/topics/[ID]/[SLUG]/[POST_ID]
$post_id = $uri->segment(4);
$topic_id = $uri->segment(2);
$post_per_page = 10;
$query = $db->query('SELECT id FROM topic_posts WHERE topic_id = ' . $topic_id . ' ORDER BY score desc');
foreach ($query as $key => $post)
{
if ($post->id == $post_id)
{
$postOnPage = ceil(($key + 1) / $post_per_page);
break;
}
}
However, the amount of posts will keep increasing and fetching all posts seems awkward.
For the date sorting I use the following query, but it's not working for the rating sorting as the post id's are then not in incrementing order:
SELECT CEIL((COUNT(*) + 1) / $posts_per_page) FROM topic_posts WHERE topic_id = $topic_id AND id < $post_id;
So... How can I avoid the php foreach loop and achieve the same with the db query?

Answer 1

I just had an idea for a completely different approach, so I add it as a second answer instead of editing the first.
For this to work predictably, you always have to include at least one unique column into your sorting as a tie breaker. I assume that many of your posts will actually have the same rating, so maybe you should for example ORDER BY score DESC, id DESC to additionally order posts with the same rating as latest first (I think this might make sense for a forum anyway).
Then for the sort order mentioned above, you can get the number of posts that sort BEFORE the post in question with the following query:
SELECT COUNT(1) FROM topic_posts
WHERE topic_id = $topic_id
AND ((score > $post_score) OR (score = $post_score AND id > $post_id));
This is a query that you can optimize using indizes etc. as needed.

Answer 2

It is possible to generate an artificial "row number" within the query, which could then be used to calculate the page the post would appear on. However, depending on your schema and table size this query might become very costly, so be sure to check the performance.
I think it is best described with a demonstration:
First the table structure and some test data:
mysql> CREATE TABLE foo (a VARCHAR(10));
mysql> INSERT INTO foo VALUES ("foo"), ("bar"), ("baz");
A query that returns all results in some order with their row numbers attached:
mysql> SELECT f.*, #rownum := #rownum+1 AS rank FROM foo f, (SELECT #rownum := 0) r ORDER BY f.a;
+------+------+
| a | rank |
+------+------+
| bar | 1 |
| baz | 2 |
| foo | 3 |
+------+------+
3 rows in set (0.00 sec)
You can then use this to select only a particular row:
mysql> SELECT * FROM (SELECT f.*, #rownum := #rownum+1 AS rank FROM foo f, (SELECT #rownum := 0) r ORDER BY f.a) t WHERE a = "foo";
+------+------+
| a | rank |
+------+------+
| foo | 3 |
+------+------+
1 row in set (0.00 sec)
Essentially, you are wrapping the inner numbering query within an outer query that only selects the wanted result row. As you can see the rank is still the same as in the "all results" case, so can now use this row number to calculate your result page.
If you want to paginate over only a subset of all records in the table (for example all posts in a particular forum), that corresponding WHERE clause goes into the inner SELECT where the ORDER BY is:
mysql> SELECT * FROM (SELECT f.*, #rownum := #rownum+1 AS rank FROM foo f, (SELECT #rownum := 0) r WHERE a != "baz" ORDER BY f.a) t WHERE a = "foo";
+------+------+
| a | rank |
+------+------+
| foo | 2 |
+------+------+
1 row in set (0.00 sec)
The downside is that it actually has to iterate over all records (well, all records that match your inner WHERE clause), so it becomes very slow with large tables.