Re: Transparent column encryption

On 21.03.23 18:47, Andres Freund wrote:
> On 2023-03-21 18:05:15 +0100, Peter Eisentraut wrote:
>> On 16.03.23 17:36, Andres Freund wrote:
>>> Maybe a daft question, but why do we need a separate type and typmod for
>>> encrypted columns? Why isn't the fact that the column is encrypted exactly one
>>> new field, and we use the existing type/typmod fields?
>>
>> The way this is implemented is that for an encrypted column, the real
>> atttypid and atttypmod are one of the encrypted special types
>> (pg_encrypted_*). That way, most of the system doesn't need to care about
>> the details of encryption or whatnot, it just unpacks tuples etc. by looking
>> at atttypid, atttyplen, etc., and queries on encrypted data behave normally
>> by just looking at what operators etc. those types have. This approach
>> heavily contains the number of places that need to know about this feature
>> at all.
>
> I get that for the type, but why do we need the typmod duplicated as well?

Earlier patch versions didn't do that, but that got really confusing
about which type the typmod really belonged to, since code currently
assumes that typid+typmod makes sense. Earlier patch versions had three
fields (usertypid, keyid, encalg), and then I changed it to (usertypid,
usertypmod, keyid) and instead placed the encalg into the real typmod,
which made everything much cleaner.