borinquenkid opened a new issue, #16746:
URL: https://github.com/apache/grails-core/issues/16746
## Problem
The code that chooses how a unique key is built or read for a nullable key
treats DB2 like H2, PostgreSQL, MySQL, MariaDB and SQL Server: a row with a
null in the key never conflicts. That is not verified for DB2 and may be wrong.
Hibernate creates DB2's unique key as `create unique index ... exclude null
keys` (`DB2Dialect#getCreateIndexTail`; `AlterTableUniqueIndexDelegate` says
DB2 does not allow unique constraints on nullable columns). As far as I know,
`exclude null keys` leaves out an entry only when all of its key columns are
null, and DB2 compares the other nulls as equal, so `('en', null)` twice would
conflict, as on Oracle. Nobody has run this on DB2.
## Where the assumption is made
- `UniqueIndexNulls` (#16701): maps only `OracleDialect` to `COMPARED`,
every other dialect, DB2 included, to `DISTINCT`. `SharedJoinTableUniqueness`
builds one unique key per association for `DISTINCT`. On DB2, if nulls are
compared, two associations that share a key column, or owners that share an
element, would be rejected on legitimate rows.
- `UniqueNullsPerDialect` for Hibernate 5 and 7 (#16583, issue #14503): DB2
reads `skip`. If DB2 compares a partly null key, `database` should read `value`
there, and the DB2 row of the table in `unique.adoc` and in the Grails 9
upgrade note is wrong.
## What is needed
1. A DB2 container spec (Testcontainers `db2` module,
`icr.io/db2_community/db2`) added to the database matrix of
`SharedJoinTableHibernate7Spec`, `SharedKeyColumnHibernate7Spec` and
`UniqueNullsHibernate7Spec`, so the behavior is measured and not read from the
source.
2. If DB2 compares partly null keys, give it the `COMPARED` / `value`
reading, with the DDL in `SharedJoinTableUniqueKeyBinderSpec` adjusted, and
correct the documentation tables.
3. Say in the documentation which databases the tests cover.
The unit-level DDL specs only show the DDL Hibernate renders. They do not
show how DB2 treats the index.
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