[myaccount80]

It is not reversible because you’re ereasing information. If you want to have a reversible addition you are not allowed to destroy the information.

The same happens if you apply the standard AND operator to two bits because the output is only one bit, therefore it is impossible to figure out the two input bits. To have a reversible AND operator you can define it like this: f(x1, x2, b) = (x1, x2, b XOR (x1 AND x2)). E.g: f(1,1,0) = (1,1,1). From this output you can get back to the input. This way you can implement a NOT gate as well, and all others gates. At the end you can implement addition using bits and your summation becomes reversible :) You can see that with this definition we are not ereasing any information, hence it becomes reversible.

[fizx]

Quantum computing is reversible in this way. Rabbit hole starts here:

https://en.wikipedia.org/wiki/Ancilla_bit

[gigatexal]

Famous last words...

After reading: “oh my god it’s full of stars!”

[codernyc16]

You’d have to do that for all results, not just one. This is basically what an event sourced system does: you store all the events and fold your state, so you can “play back” to any point in time. You could theoretically reverse from the end, but I haven’t seen it done.

[setr]

Urbit ;-)

[Zenst]

A transactional database would tick that box, events are stored and can rollback to a point in time.

[acoma]

Not quite. Dropping a column or updating a row is typically irreversible.

[Zenst]

That does depend upon the database, but can be done in some without resorting to a backup.

[continuational]

In PostgreSQL, schema changes are transactional.

[anarazel]

> In PostgreSQL, schema changes are transactional.

With a few exceptions (that refuse to run in an explicit transaction): E.g. {CREATE, DROP} DATABASE, {CREATE, DROP} TABLESPACE, {CREATE INDEX, DROP INDEX, REINDEX} CONCURRENTLY, REINDEX {SCHEMA, SYSTEM, DATABASE}.

[zcrackerz]

Also, try adding a value to an enum via ALTER TYPE ... ADD VALUE