INSERT IGNORE WITH INDEX vs INSERT UNIQUE VALUES in mysql - mysql

Question

I have the following table:
CREATE TABLE `Triples` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`Subject` longtext COLLATE utf8mb4_unicode_ci,
`Predicate` longtext COLLATE utf8mb4_unicode_ci,
`Object` longtext COLLATE utf8mb4_unicode_ci,
`SubHash` binary(16) DEFAULT NULL,
`PredHash` binary(16) DEFAULT NULL,
`ObHash` binary(16) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `PredHash` (`PredHash`),
KEY `ObHash` (`ObHash`),
KEY `SubHash` (`SubHash`)
) ENGINE=InnoDB
It contains about 800 Million rows.
Now I want to create two other tables.
One table is Nodes:
CREATE TABLE `Nodes` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`val_hash` binary(16) NOT NULL,
`val` longtext COLLATE utf8mb4_unicode_ci NOT NULL,
`subjectCount` bigint(20) unsigned NOT NULL DEFAULT '0',
`objectCount` bigint(20) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`id`)
)
That will store all Subjects and Objects from Triples one time (val_hash will become a Key after the data is inserted).
The Question is which of the INSERTs performs better?:
INSERT INTO Nodes(val,val_hash)
SELECT j.val,j.val_hash FROM (SELECT Subject as val, SubHash as val_hash
FROM Triples GROUP BY val_hash
UNION
SELECT Object as val, ObHash as val_hash
FROM Triples GROUP BY val_hash) as j
GROUP BY j.val_hash
OR The following:
ALTER TABLE Nodes
ADD UNIQUE KEY(val_hash);
INSERT IGNORE INTO Nodes(val,val_hash)
SELECT Subject,SubHash FROM Triples GROUP BY SubHash;
INSERT IGNORE INTO Nodes(val,val_hash)
SELECT Object,ObHash FROM Triples GROUP BY ObHash;
I ask because a key adds complexity to inserts but a union requires a temporary table and i don't know which of both is better in this situation.