Does Postgres support nested or autonomous transactions?

Postgres does support nested transactions, but they differ from the conventional SQL, more like transactions with nested partial points.

On the top level you always have your typical BEGIN/COMMIT/ROLLBACK, and on nested levels you have to use the following commands:

• SAVEPOINT name – creates a new savepoint, with name unique for the transaction
• RELEASE SAVEPOINT name – commits the savepoint, though it will only persist if the containing transaction commits
• ROLLBACK TO SAVEPOINT name – rolls back the savepoint

You would also have to make sure that:

• The names used for each SAVEPOINT are unique;
• Failure in one SAVEPOINT is propagated upwards to the top level.

The last bit is a bit tricky, unless you use a library that can do that for you automatically.

When I wrote pg-promise, I made sure that those two provisions are guaranteed:

• It generates save-point names automatically, as sp_xy, where x is the current task/transaction depth, and y is the actual transaction level + 1;
• It executes containing ROLLBACK TO SAVEPOINT name, plus the top-level ROLLBACK in case a child transaction fails – all built on the standard promise-chaining logic.

See also the limitations of the PostgreSQL nested transactions explained…