Types: Adjust CrateJsonEncoder to cast Decimal values to Python float types

[amotl]

About

This patch may be needed to fix value marshalling behaviour for Decimal types. It is apparently needed to be compatible with PostgreSQL.

This came up when using the SQLAlchemy dialect on behalf of a Singer/Meltano database sink component, in order to satisfy the software tests, where both implementation and test cases have been derived from the corresponding PostgreSQL adapter.

Status

Not sure if it classifies as a bug, but indeed it feels like a bug if the driver deviates from the standard DBAPI/PostgreSQL behaviour. Based on further investigations, the patch needs to find the right slot in one way or another.

References

• This is coming from a monkeypatch to meltano-target-cratedb.
• It is related to Types: Add method ObjectArray.as_generic crate-workbench/sqlalchemy-cratedb#23 and Types: Improve type mappings crate-workbench/sqlalchemy-cratedb#24.

Backlog

• Adjust software tests.
• Just a short note about type support within the documentation.
• Changelog item. In this case, it is a BREAKING CHANGE, because, beforehand, values of Decimal type have been cast to Python's str type.

[seut]

I think using a float won't work as a Decimal has a 128bit range and thus cannot fit into a float.