Neo4j - 按照发布日期的顺序获取 Post 节点列表

Question

I've recently started sketching up a personal project which will involve a social network side. I have some professional experience with Neo4j and while it feels like a perfect match there is one query that concerns me.

Imagine a general social network: users follow each other, users post posts, users can see the posts written by the users they're following. This is cleanly expressed in Neo4j through :User and :Post labelled nodes, connected through :posted and :follows relationships.

So I could get the posts by users I follow using a query like:

MATCH (:User {user_id: 1})-[:follows]->(:User)-[:posted]->(p:Post)
RETURN p

This is pretty clean and simple. My concern is that realistically I will want to get the most recent 10 posts, and then the 10 posts after that, and so on.

So I created an index on a created_at field in :Post nodes and added an ORDER BY p.created_at DESC clause to the query. I thought this would allow me to sort them efficiently however running an EXPLAIN on this query shows that ORDER BY clauses do not, for the most part, use indexes to speed up this process. As such I'm unsure if there's a way to get these efficiently when the result set becomes significantly large.

This may be inexperience or just approaching this data model incorrectly. Can I get some input on this kind of problem? Should I model my data differently? Is my query/index wrong? Is there something I'm missing? How would you do this?

EDIT 1: Example query for something like what I meant:

MATCH (:User {user_id: 1})-[:follows]->(:User)-[:posted]->(p:Post)
RETURN p
ORDER BY p.created_at DESC
LIMIT 10

Also I've been thinking that using a range (in a WHERE clause) is a possibility to limit the result set size but still unsure of whether there's a better way?

EDIT 2 (Solution): This was the final query that made the Cypher planner use the index for this problem:

MATCH (:User {user_id: 1})-[:follows]->(:User)-[:posted]->(p:Post)
USING INDEX p:Post(created_at)
WHERE p.created_at < datetime()
RETURN p
ORDER BY p.created_at DESC
LIMIT 10

Answer 1

Neo4j 3.5 introduced some support for using indexes to perform ORDER BY operations , with some restrictions.

But, currently (in neo4j 3.5.3), even when the usage of an index is supported for ORDER BY , the Cypher planner does not seem to automatically use it for that purpose. In my experimentation with version 3.5.3, I found that if you do not use the index in a WHERE clause then the planner will not use the index at all.

So, as a simple workaround, you can just add a trivial WHERE clause using the index. For example, here is a modified version of your query that will "trick" the planner into using the index for ORDER BY :

MATCH (:User {user_id: 1})-[:follows]->(:User)-[:posted]->(p:Post)
WHERE p.created_at > 0
RETURN p
ORDER BY p.created_at DESC
LIMIT 10