Working on the pagination feature for the Dgraph tech-demo I discovered two major flaws.
Linked issues: #3472 and #3473
Initial database state: there’s 83.719 post nodes in Dgraph:
Post.id is indexed with exact. Getting the last 6 posts ordered by Post.id returns [0x78b52, 0x68678, 0x59722, 0x726ff, 0x65dbd, 0x78324]
1. first and after don’t work together:
I failed implementing cursor-based pagination because first and after don’t seem to work in combination. I’d expect this query:
{
pageAfter(
func: has(Post.id),
orderdesc: Post.id,
first: 3,
after: 0x59722
) {
uid
Post.id
Post.title
}
}
to return uids: 0x726ff, 0x65dbd, 0x78324 since the predicate is sorted by Post.id but as you can see the actual results ([0x5aefe, 0x73133, 0x74920]) are very different. Is this a bug?!
2. offset based pagination is very slow
Later I tried implementing an offset` based pagination but it turned out to be very slow:
{
pageAfter(
func: has(Post.id),
orderdesc: Post.id,
first: 3,
offset: 83716
) {
uid
Post.id
Post.title
}
}
Getting the last 3 posts in the list ordered by Post.id takes 2.5 - 4 seconds which is nuts!
I’ve tried a hash index for Post.id which is slighly faster but still takes 1.5 - 2 seconds!
Dgraph seems to be doing “full-table-scans” all the time even though Post.id is indexed and unique (which the database doesn’t know since there’s no unique constraint).
Is there anyway to get pagination right?! 