Create relationship between each two nodes for a set of nodes - neo4j

Question

I have created many nodes in neo4j, the attributes of these nodes are the same, they all have user_id and item_id, the code used is as follows:
LOAD CSV WITH HEADERS FROM 'file://data.csv' AS row
CREATE (main:Main_table {USER_ID: row.user_id,
ITEM_ID: row.item_id}
)
CREATE INDEX ON :Main_table(USER_ID);
CREATE INDEX ON :Main_table(ITEM_ID);
Now I want to create relationship between the nodes with the same user_id or item_id. For example, if node A, B and C have the same USER_ID, I want to create (A)-[:EDGE]->(B), (A)-[:EDGE]->(C) and (B)-[:EDGE]->(C). In order to achieve this goal, I tried the following code:
MATCH (a:Main_table),(b:Main_table)
WHERE a.USER_ID = b.USER_ID
CREATE (a)-[:USER_EDGE]->(b);
MATCH (a:Main_table),(b:Main_table)
WHERE a.ITEM_ID = b.ITEM_ID
CREATE (a)-[:ITEM_EDGE]->(b);
But due to the large amount of data (3000000 nodes, 100000 users), this process is very slow, how can I quickly complete this process? Any help would be greatly appreciated!

Answer 1

Your query is causing a cartesian product, and the Cypher planner does not use indexes to optimize node lookups involving node property comparisons.
A query like this (instead of your USER_EDGE query) may be faster, as it does not cause a cartesian product:
MATCH (a:Main_table)
WITH a.USER_ID AS id, COLLECT(a) AS mains
UNWIND mains AS a
UNWIND mains AS b
WITH a, b
WHERE ID(a) < ID(b)
MERGE (a)-[:USER_EDGE]->(b)
This query uses the aggregating function COLLECT to collect the nodes that have the same USER_ID value, and uses the ID(a) < ID(b) test to ensure that a and b are not the same nodes and to also prevent duplicate relationships (in opposite directions).