Liam Clarke ( https://hibernate.atlassian.net/secure/ViewProfile.jspa?accountId=557058%... ) *commented* on HHH-10683 ( https://hibernate.atlassian.net/browse/HHH-10683?atlOrigin=eyJpIjoiZWI1ZD... ) Re: Pooled id generation optimizer is causing ids from sequences to be reused ( https://hibernate.atlassian.net/browse/HHH-10683?atlOrigin=eyJpIjoiZWI1ZD... ) I’ve just hit this issue in Hibernate 5.4.12 Final (upgraded from Hibernate 4.3.x) against Postgres, same symptom - Hibernate attempts to insert a PK that is the DBs’s nextval - allocationSize (which defaults to 50 on the JPA @SequenceGenerator annotation). We didn’t have the same issue with Hibernate 4.3 insert into BACKFILL (HTML, IS_USED, ID) values (NULL, 'FALSE', 12029) was aborted: ERROR: duplicate key value violates unique constraint ... psql (11.5 (Debian 11.5-3.pgdg90+1)) Type "help" for help. test=> select nextval('backfill_seq'); nextval --------- 12079 (1 row) Setting hibernate.id.optimizer.pooled.prefer_lo to true as mentioned by the original reporter also fixed the issue for me. I will try to attach a unit test to reproduce when I get time. ( https://hibernate.atlassian.net/browse/HHH-10683#add-comment?atlOrigin=ey... ) Add Comment ( https://hibernate.atlassian.net/browse/HHH-10683#add-comment?atlOrigin=ey... ) Get Jira notifications on your phone! Download the Jira Cloud app for Android ( https://play.google.com/store/apps/details?id=com.atlassian.android.jira.... ) or iOS ( https://itunes.apple.com/app/apple-store/id1006972087?pt=696495&ct=Em... ) This message was sent by Atlassian Jira (v1001.0.0-SNAPSHOT#100122- sha1:52999ef )