Efficient way to create relationships from a csv in neo4j - graph

Question

I am trying to populate my graph db with relationships that I currently have access to in a file.
They are in the form were each line in the relationship csv has the unique IDs of the two nodes that relationship is describing as well as the kind of the relationship it is.
Each line in the relationship csv has something along of the lines of:
uniqueid1,uniqueid2,relationship_name,property1_value,..., propertyn_value
I already had all nodes created and was working on matching the nodes that match the uniqueids specified in each of the files and then creating the relationship between them.
However, the tend to be taking a long time to be creating for each of the relationships and my suspicion is that I am doing something wrong.
The csv file has about 2.5 million lines with different relationship types. So i manually set the relationships.rela property to one of them and try to run through creating all nodes involved in that relationship and follow up with the next using my where clause.
The amount of properties each node has has been reduced by an ellipsis(...) and the names redacted.
I currently have the query to create the relationships set up in the following way
:auto USING PERIODIC COMMIT 100 LOAD CSV WITH HEADERS FROM 'file:///filename.csv' as relationships
WITH relationships.uniqueid1 as uniqueid1, relationships.uniqueid2 as uniqueid2, relationships.extraproperty1 as extraproperty1, relationships.rela as rela... , relationships.extrapropertyN as extrapropertyN
WHERE relations.rela = "manager_relationship"
MATCH (a:Item {uniqueid: uniqueid1})
MATCH (b:Item {uniqueid: uniqueid2})
MERGE (b) - [rel: relationship_name {propertyvalue1: extraproperty1,...propertyvalueN: extrapropertyN }] -> (a)
RETURN count(rel)
Would appreciate if alternate patterns could be recommended.

Answer 1

Indexing is a mechanism that databases use, to speed up data lookups. In your case, since Item nodes are not indexed, these two matches can take a lot of time, especially if the number of Item nodes is very large.
MATCH (a:Item {uniqueid: uniqueid1})
MATCH (b:Item {uniqueid: uniqueid2})
To speed this up, you can create an index on Item nodes uniqueid property, like this:
CREATE INDEX unique_id_index FOR (n:Item) ON (n.uniqueid)
When you'll run your import query after creating the index, it will be much faster. But it will still take a bit of time as there are 2.5 million relationships. Read more about indexing in neo4j here.

Answer 2

Aside from suggestion from Charchit about create an index, I will recommend using an APOC function apoc.periodic.iterate which will execute the query into parallel batches of 10k rows.
https://neo4j.com/labs/apoc/4.4/overview/apoc.periodic/apoc.periodic.iterate/
For example:
CALL apoc.periodic.iterate(
"LOAD CSV WITH HEADERS FROM 'file:///filename.csv' as relationships RETURN relationships",
"WITH relationships.uniqueid1 as uniqueid1, relationships.uniqueid2 as uniqueid2, relationships.extraproperty1 as extraproperty1, relationships.rela as rela... , relationships.extrapropertyN as extrapropertyN
WHERE relations.rela = "manager_relationship"
MATCH (a:Item {uniqueid: uniqueid1})
MATCH (b:Item {uniqueid: uniqueid2})
MERGE (b) - [rel: relationship_name {propertyvalue1: extraproperty1,...propertyvalueN: extrapropertyN }] -> (a)",{batchSize:10000, parallel:true})
The first parameter will return all the data in the csv file then it will divide the rows into 10k per batch and it will run it in parallel using default concurrency (50 workers).
I use it often where I load 40M nodes/edges in about 30mins.