HQL's insertion conflict clause accepts a constraint name: insert ... on conflict on constraint uk_book_title do nothing (and do update set ...).

The SQL AST delivers only the name: ConflictClause.getConstraintName() returns it and getConstraintColumnNames() is empty. Translating to MQL needs the columns behind the name, because the upsert filter is equality on the conflict-target columns.

on constraint <name> is exactly equivalent to naming that constraint's columns (verified against PostgreSQL: the conflict-detection scope and do update support are identical), so resolving the name reduces these statements to the already-supported field-list forms with no new translation mechanism.

The columns live in boot-time metadata only (org.hibernate.mapping.UniqueKey); the work is capturing them at bootstrap into something the translator can consult at runtime, keyed by constraint name.