Figure out what people actually use `derive(PostgresType)` for

[workingjubilee]

A lot of programmers using pgrx seem to think derive(PostgresType) is required for some things. However, they aren't very clear on what those things are. For instance, they also think derive(Serialize, Deserialize) is required for implementing FromDatum and IntoDatum, but it is not! You can implement those by hand. My PR¹ does propose to do away with that ability, leaving a gap for those who do use by-hand implementations that we should close before releasing 0.12.0.

Frankly, I myself am not entirely sure what derive(PostgresType) is actually... for? Not because I can't read the code, but because it does a lot of small things, currently, which are seemingly-needlessly coupled. I don't know which one was the "main point". It would be nice if we could work out a more consistent story for it.

We need

• to figure out everything PostgresType does
  • it emits DDL for CREATE TYPE
  • it potentially implements JsonInOutFuncs
  • it potentially implements PgVarlenaInOutFuncs
  • it potentially implements FromDatum and IntoDatum
  • it may be overridden by InOutFuncs
  • it emits a bunch of #[pg_extern] functions for each of these cases
• to figure out what parts of PostgresType do actually need to be coupled
• to figure out if there's any use-cases we're missing
• a substitute for derive(PostgresType) + impl {From,Into}Datum (sortof)
• a substitute for derive(PostgresType) + impl InOutFuncs
• a substitute for derive(PostgresType) + impl InOutFuncs + impl {From,Into}Datum

Footnotes

1. https://github.com/pgcentralfoundation/pgrx/pull/1381 ↩

[workingjubilee]

We don't necessarily need to address all possible concerns here but we would need to figure out enough that the 0.12.0 migration becomes possible for our extant users if we accept #1381

[workingjubilee]

Okay, so the primary effect of derive(PostgresType) is that it emits CREATE TYPE DDL, which is otherwise more annoying to handle correctly, as it requires knowing SQL in order to correctly write it and comes with implicit unsafe obligations. It's important that programmers not be expected to know both Rust and SQL AND the nuances of Postgres in order to use pgrx.

[eeeebbbbrrrr]

Okay, so the primary effect of derive(PostgresType) is that it emits CREATE TYPE DDL.

It also emits a ton of code, including FromDatum/IntoDatum impls and some of the Postgres type support functions for the text representation of the type-as-a-value.