Best use of CREATE CONSTRAINT during LOAD CSV - neo4j

Question

I'm unsure if I'm using CREATE CONSTRAINT optimally while importing CSV data via LOAD CSV and would appreciate feedback/advice from the more knowledgeable.
I am importing from databases of about 3 and 12 million records. I know that the bulk import function would be faster, but for various reasons, LOAD CSV is the better option for this project. I can let things run for a long time, but want to be sure I'm optimizing as much as possible.
My code is currently:
CREATE CONSTRAINT ON (i:Inventor) ASSERT i.hanID IS UNIQUE;
CREATE CONSTRAINT ON (p:Patent) ASSERT p.patNo IS UNIQUE;
CREATE CONSTRAINT ON (c:Country) ASSERT c.countryCode IS UNIQUE;
// Import Inventors and link them to their country
USING PERIODIC COMMIT
LOAD CSV WITH HEADERS FROM "file:///...//names.short" AS row
FIELDTERMINATOR '|'
MERGE (c:Country {countryCode:row.Person_ctry_code})
MERGE (i:Inventor {hanId:row.HAN_ID, name:row.Person_name_clean})
CREATE (i)-[:LivesIn]->(c);
// Load patents and link the to their inventors
USING PERIODIC COMMIT
LOAD CSV WITH HEADERS FROM "file:///.../patents.short" as row
FIELDTERMINATOR '|'
MERGE (i:Inventor {hanId:row.HAN_ID})
MERGE(p:Patent {patNo:row.Patent_number})
CREATE (i)-[:Invented]->(p);
Each inventor has a unique hanID, each patent a unique patNo and each country a unique countryCode, although each inventor, patent and country may show up in the data many times.
Is creating the constraints before I begin the LOAD CSV statements optimal?
Are there any obvious ways to improve the speed of my imports?
Thank you very much.

Answer 1

Constraint creation before loading CSV is a good move, as constraints only need to be created once.
As for your import queries, it's best to MERGE only with the unique property, and use ON CREATE to SET additional properties (like an inventor's name).
As far as speed improvements go, when you're importing you're likely only doing this once, so speed usually isn't a factor unless it's taking an unusually long time for some reason.
One way you could improve this is to load CSVs with just :Country, just :Inventor, and just :Patent, with no repeats of entries, and use CREATE instead of MERGE to get them into the db. Then, after all nodes are imported, you can use the queries and CSVs in your description to create relationships, but you can use use MATCH instead of MERGE on all nodes.
Remember that MERGE is shorthand for attempting a MATCH, and if no MATCH, it will CREATE, so creating all your nodes ahead of time with CREATE avoids the extra unnecessary checks to see if the node exists first.
EDIT
cybersam's answer for a different question highlighted something I wasn't previously aware of. Apparently indexes are not used for lookup when using another property for input (that should apply to unique properties too).
To get around this, you'll have to alias the properties as values, then use those.
For example, in your query to load :Patent and :Inventor nodes, you would have to do something like this:
USING PERIODIC COMMIT
LOAD CSV WITH HEADERS FROM "file:///.../patents.short" as row
FIELDTERMINATOR '|'
WITH row.HAN_ID as hanId, row.Patent_number as patNo
MERGE (i:Inventor {hanId:hanId})
MERGE(p:Patent {patNo:patNo})
CREATE (i)-[:Invented]->(p);
CREATE (i)-[:LivesIn]->(c);