Paging in SQL with LIMIT/OFFSET sometimes results in duplicates on different pages

Question

I'm developing an online gallery with voting and have a separate table for pictures and votes (for every vote I'm storing the ID of the picture and the ID of the voter). The tables related like this: PICTURE <--(1:n, using VOTE.picture_id)-- VOTE. I would like to query the pictures table and sort the output by votes number. This is what I do:

SELECT
    picture.votes_number,
    picture.creation_date,
    picture.author_id,
    picture.author_nickname,
    picture.id,
    picture.url,
    picture.name,
    picture.width,
    picture.height,
    coalesce(anon_1."totalVotes", 0)
FROM picture
LEFT OUTER JOIN
    (SELECT
        vote.picture_id as pid,
        count(*) AS "totalVotes"
     FROM vote
     WHERE vote.device_id = <this is the query parameter> GROUP BY pid) AS anon_1
ON picture.id = anon_1.pid
ORDER BY picture.votes_number DESC
LIMIT 10
OFFSET 0

OFFSET is different for different pages, of course.

However, there are pictures with the same ID that are displayed on the different pages. I guess the reason is the sorting, but can't construct any better query, which will not allow duplicates. Could anybody give me a hint?

Thanks in advance!

Answer 1

Do you execute one query per page to display? If yes, I suspect that the database doesn't guarantee a consitent order for items with the same number of votes. So first query may return { item 1, item 2 } and a 2nd query may return { item 2, item 1} if both items have same number of votes. If the items are actually items 10 and 11, then the same item may appear on page 1 and then on page 2.

I had such a problem once. If that's also your case, append an extra clause to the order by to ensure a consistent ordering of items with same vote number, e.g.:

ORDER BY picture.vote, picture.ID

Answer 2

The simples explanation is that you had some data added or some votes occured when you was looking at different pages.

I am sure if you would sorte by ID or creation_date this issue would go away.

I.e. there is no issue with your code