Skiping relationship creation if already exist, not about MERGE - neo4j

Question

I am new to neo4j, my data is in csv files trying load them in db and create relationships.
departments.csv(9 rows)
dept_name
dept_no
dept_emp.csv(331603 rows)
dept_no
emp_no
from_date
to_date
I have create nodes with labels departments and dept_emp with all columns as properties. now trying to create relationship between them.
CALL apoc.periodic.iterate("
load csv with headers from 'file:///dept_emp.csv' as row return row",
"match(de:dept_emp)
match(d:departments)
where de.dept_no=row.dept_no and d.dept_no= row.dept_no
merge (de)-[:BELONGS_TO]->(d)",{batchSize:10000, parallel:false})
I do have indexes on :dept_emp and :departments
When I try to run this it is taking ages to complete(many days). When I change the batch size to 10 it created 331603 relations, but it kept on running until it completes all the batches which is taking too long. When it encounters 9 different dept_no at initial rows in dept_emp.csv it is creating all the relations but it has to complete all the batches. In each batch it has to scan all the 331603 relations which were create in first two batches or so. Please help me with optimizing this.
Here I have used apoc.periodic.iterate to deal with the huge data in future, here how the data is related and how I am trying to establish the relation is making the problem . Each department will be having many dept_emp nodes connected.
Currently using Neo4j 4.2.1 version
Max heap size is 1G due to my laptop limitations.

Answer 1

There's no need to create nodes in this fashion, i.e. set properties and then load the same csv again but match all nodes in the graph and do a cartesian join.
Instead:
USING PERIODIC COMMIT 500
LOAD CSV WITH HEADERS FROM 'file:///departments.csv' AS row
CREATE (d:Department) SET d.deptNo=row.dept_no, d.name=row.dept_name
USING PERIODIC COMMIT 500
LOAD CSV WITH HEADERS FROM 'file:///dept_emp.csv' AS row
MATCH (d:Department {deptNo:row.`dept_no`})
WITH d
MERGE (e:Employee {empNo: row.`emp_no`})
MERGE (e)-[:BELONGS_TO]->(d)