Deep merge takes too long

[mrbinwin]

Hello!

service api v.5.5.0

I run a deep merge of 2 launches:
launch 1: 6274 test_items
launch 2: 3204 test_items

It takes 1750 sec (29 minutes) to merge them.
It happens because of this query:
https://github.com/reportportal/service-api/blob/develop/src/main/java/com/epam/ta/reportportal/core/item/identity/TestItemUniqueIdGenerator.java#L87

The merge script pulls from db all parents for each test_item of each of the merging launches.
So in my case it sends 9478 database queries and each query pulls 3 entity.
Most of the time this script pulls the same test_items (the same suite, story, parent step).
Because of usage of findAllById() method Hibernate can not use any kind of cache to improve the performance of these queries.

I've added a simple HashMap cache to check if it can work faster.

Deep merge without caching:
9478 db queries
1750 sec

With cache:
366 db queries
366 item names cached
27690 cache hits
35 sec

Long merges can be a problem because during high load a merge procees locks other database queries and it can lead to API connection pool overflow and this error:
Caused by: java.sql.SQLTransientConnectionException: HikariPool-1 - Connection is not available, request timed out after 30000ms.