# id-generation

Companion module for the ankurm.com article **Hibernate ID Generation: IDENTITY vs SEQUENCE vs UUID v7, and Why Batch Inserts Break**
(https://ankurm.com/hibernate-id-generation-identity-sequence-uuid-v7-batch-inserts/).
Every claim in the article comes from a test here; each test writes what it observed to `output/` and asserts the same facts.

## Versions

| Component | Version | Notes |
|---|---|---|
| Hibernate ORM | `7.4.14.Final` | newest 7.x GA on Maven Central on 2026-11-05 (`8.0.0.Beta4` is listed but is not GA). Overrides the `7.4.5.Final` that Spring Boot 4.1.1 manages, same as `soft-delete` |
| Spring Boot parent | `4.1.1` | same parent as the sibling modules (`4.1.2` was already on Maven Central; not adopted so the modules stay comparable) |
| uuid-creator | `6.1.1` | only for one comparison generator |
| H2 | `2.4.240` | in memory, plus file databases for the index-size test |
| PostgreSQL | `17` (`postgres:17-alpine`) | Testcontainers. **Not run for the committed transcripts**, see below |
| Java | `25` (Temurin/Oracle 25.0.3) | |

## Run

```bash
./scripts/run-all.sh      # javap dump + mvn clean test; regenerates everything in output/
```

Tests that can use PostgreSQL run against it when a Docker daemon answers and otherwise write `SKIPPED: no Docker daemon ...` into the
transcript. **The committed transcripts were produced without Docker, so every PostgreSQL section says SKIPPED and the PostgreSQL code paths
(including `06b-index-size-postgres.txt`: `pg_relation_size`, `pgstatindex`) have not been executed.** Run the script on a machine with Docker to fill them in.

## Output

| File | What it shows |
|---|---|
| `00-javap.txt` | `javap` of `UuidGenerator`, `UuidGenerator$Style`, `UuidValueGenerator`, `UuidVersion7Strategy`, `SequenceMismatchStrategy`, `StandardOptimizerDescriptor` and the static members of `java.util.UUID` |
| `00b-style-by-version.txt` | the `Style` constants in hibernate-core 6.5.2, 6.6.8, 7.0.0, 7.4.0, 7.4.14 (hand-run `javap`, not regenerated by the script) |
| `01-uuid-styles.txt` | version nibble, variant and ordering of ids from every `Style`, the JDK's UUID factories, what `Style.TIME` really contains |
| `01b-uuid-generation-rate.txt` | in-memory cost of generating one id per generator (indicative) |
| `02-identity-vs-sequence.txt` | DDL per strategy, IDENTITY insert at `persist()`, sequence calls for `allocationSize` 1 vs 50, TABLE generator statements |
| `02b-default-sequence-naming.txt` | default sequence names and the `legacy` / `single` naming switch |
| `03-batch-inserts.txt` | 10,000 inserts per strategy: prepared statements, `addBatch`, `executeBatch` counts, indicative timings |
| `03b-order-inserts.txt` | `hibernate.order_inserts` on a parent/child graph |
| `04-sequence-increment-mismatch.txt` | mapping `allocationSize=50` over a sequence `INCREMENT BY 1`, under each mismatch strategy |
| `04b-sequence-step-bigger-than-mapping.txt` | mapping `allocationSize=1` over a sequence `INCREMENT BY 50` |
| `05-auto-and-uuid-per-dialect.txt` | what `@GeneratedValue` and UUID columns become on H2, PostgreSQL, MySQL, MariaDB, Oracle, SQL Server (DDL only) |
| `06-index-size-h2.txt` | 200,000 rows per key type in an H2 file database: file size |
| `06b-index-size-postgres.txt` | the same on PostgreSQL (`SKIPPED` without Docker) |
| `07-spring-data-save.txt` | `save()` on assigned vs generated ids: which ones cost a `SELECT` first |

## Not covered (not run)

MySQL / InnoDB clustered-index behaviour, SQL Server, Oracle (DDL only), PostgreSQL index measurements, multi-node clock skew, `hibernate.id.optimizer.pooled.preferred` variants.