Tag: NPlusOne

All the articles with the tag "NPlusOne".

• JPA N+1 and the Four JOIN FETCH Traps — MultipleBagFetchException, Pagination OOM, OneToOne LAZY

  4 May, 2026

  In a 4-depth domain (owner→merchant→rule→history) findAll + child traversal yields 121 SQL. JOIN FETCH collapses it to 1 (12× faster). Fetching two collections at once raises MultipleBagFetchException — Hibernate refuses the cartesian of two Bags. JOIN FETCH + setMaxResults emits HHH000104 and applies pagination *in memory* — silent OOM at scale. Non-owning @OneToOne LAZY is *always fetched* because the proxy cannot tell whether the value is null. **`default_batch_fetch_size: 10` reduces N+1 from 121 → 13 prep (9.3× drop) — but does NOT fix @OneToOne non-owning LAZY (still 1201 prep)**, because batch fetch only batches collection LAZY triggers, not row-by-row ToOne SELECTs. The fetch traps come from Bag/List/Set semantics, proxy limitations, and Hibernate's cartesian handling — JOIN FETCH alone is half the answer.