Return Generated ID in R2dbc Insert

When using Postgres, it is easy to add RETURNING id to attach the generated ID in the FetchSpec and extract it.

But when using H2 and Spring R2dbc, there is no built-in RETURNING support as Postgres.

The Jdbc also includes PrepareStatement.getGeneratedKeys() to fetch the generated key values. Spring Jdbc also include simple GeneratedKeyHolder to fetch the generated ID in the insert clause.

Spring Data R2dbc issues also mentioned it has resolved this issue, https://github.com/spring-projects/spring-data-r2dbc/issues/14.

But the raw R2dbc API includes a Statement.returnGeneratedValues, but it is not adopted in the Spring R2dbc.

I wish there is a general solution for all database. When inserting a record, no need to add database specific RETURNING clause, provide a Spring Jdbc similar KeyHolder to store the generated keys in the query execution, and easily extract the key from the FetchSpec.

Start with the R2DBC Statement.returnGeneratedValues API and Spring R2DBC's insert and FetchSpec execution paths. The issue names no Spring Framework files or tests; done would require a decided, database-independent way to request, retain, and extract generated keys, including H2 and PostgreSQL behavior.