Mark a transaction uncommittable

This is a proposal for a new transaction characteristic. I haven't
written any code, yet, and am interested in hearing if others may find
this feature useful.

Many a times we start a transaction that we never intend to commit;
for example, for testing, or for EXPLAIN ANALYZE, or after detecting
unexpected results but still interested in executing more commands
without risking commit, etc.

A user would like to declare their intent to eventually abort the
transaction as soon as possible, so that the transaction does not
accidentally get committed.

This feature would allow the user to mark a transaction such that it
can never be committed. We must allow such marker to be placed when
the transaction is being started, or while it's in progress.

Once marked uncommittable, do not allow the marker to be removed.
Hence, once deemed uncommittable, the transaction cannot be committed,
even intentionally. This protects against cases where one script
includes another (e.g. psql's \i command), and the included script may
have statements that turn this marker back on.

Any command that ends a transaction (END, COMMIT, ROLLBACK) must
result in a rollback.

All of these properties seem useful for savepoints, too. But I want to
focus on just the top-level transactions, first.

I feel like the BEGIN and SET TRANSACTION commands would be the right
places to introduce this feature.

BEGIN [ work | transaction ] [ [ NOT ] COMMITTABLE ];
SET TRANSACTION [ [ NOT ] COMMITTABLE ];

I'm not yet sure if the COMMIT AND CHAIN command should carry this
characteristic to the next transaction.

Thoughts?

Best regards,
Gurjeet https://Gurje.et
http://aws.amazon.com