Will this type of pagination scale? - performance

Question

I need to paginate on a set of models that can/will become large. The results have to be sorted so that the latest entries are the ones that appear on the first page (and then, we can go all the way to the start using 'next' links).
The query to retrieve the first page is the following, 4 is the number of entries I need per page:
SELECT "relationships".* FROM "relationships" WHERE ("relationships".followed_id = 1) ORDER BY created_at DESC LIMIT 4 OFFSET 0;
Since this needs to be sorted and since the number of entries is likely to become large, am I going to run into serious performance issues?
What are my options to make it faster?
My understanding is that an index on 'followed_id' will simply help the where clause. My concern is on the 'order by'

Answer 1

Create an index that contains these two fields in this order (followed_id, created_at)
Now, how large is the large we are talking about here? If it will be of the order of millions.. How about something like the one that follows..
Create an index on keys followed_id, created_at, id (This might change depending upon the fields in select, where and order by clause. I have tailor-made this to your question)
SELECT relationships.*
FROM relationships
JOIN (SELECT id
FROM relationships
WHERE followed_id = 1
ORDER BY created_at
LIMIT 10 OFFSET 10) itable
ON relationships.id = itable.id
ORDER BY relationships.created_at
An explain would yield this:
+----+-------------+---------------+------+---------------+-------------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------+------+---------------+-------------+---------+------+------+-----------------------------------------------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables |
| 2 | DERIVED | relationships | ref | sample_rel2 | sample_rel2 | 5 | | 1 | Using where; Using index |
+----+-------------+---------------+------+---------------+-------------+---------+------+------+-----------------------------------------------------+
If you examine carefully, the sub-query containing the order, limit and offset clauses will operate on the index directly instead of the table and finally join with the table to fetch the 10 records.
It makes a difference when at one point your query makes a call like limit 10 offset 10000. It will retrieve all the 10000 records from the table and fetch the first 10. This trick should restrict the traversal to just the index.
An important note: I tested this in MySQL. Other database might have subtle differences in behavior, but the concept holds good no matter what.

Answer 2

you can index these fields. but it depends:
you can assume (mostly) that the created_at is already ordered. So that might by unnecessary. But that more depends on you app.
anyway you should index followed_id (unless its the primary key)