🚩 "CREATE FUNCTION" statement | PL/pgSQL support (big feature) about pg-mem

[oguimbal]

CREATE FUNCTION statements are now partially supported in [email protected], via the registration of custom languages.

This allows to provide either a custom compiler (for custom scripts such as javascript via eval()), or a mock implementation for each function creation.

(I dont think actual plpgsql support is coming anytime soon)

[RodolfoSilva]

Supporting the "create function" statement, without actually parsing/compiling the function body (which would force you to declare mocks for your created pg functions directly in JS).

Hey @oguimbal what's the path that you suggest to implement this? I think this is most easy way to do this right now. Allowing the stub implementation of PSQL functions like extensions.

[oguimbal]

The path... you mean how it must be implemented ?

I've been wanting to add that in contributing.md for a while, so i'll do that now.

Like any feature of pg-mem, it breaks down into:

A) Implement its parser

NB: For this use case (create function stubs), this will not exactly look like below, because create function body must be "ignored" ... thus, I think that the lexer must be modified. Which is a bit tricky. But otherwise, it goes like that:

1. Look very hard at the statement syntax
2. Add the target statement type in pgsql-ast-parser AST based on what I understood.
3. Implement unit tests I want to pass (ex with create sequence UTs)
4. Implement its syntax in a new nearley file (example with the create sequence statement) and reference it in main.ne
5. Work on it until unit tests are all failing, but with an error which is not a parsing error
6. Add an entry for the new statement in IAstPartialMapper here and add its default visitor implem like here and here until compiler stops screaming
7. Add the "toSql" implementation of the new statement like here (until compiler stops)
8. Check that ALL unit tests are green (not only the new ones)
9. Release pgsql-ast-parser

B) Executor

1. install pgsql-ast-parser@latest
2. Write unit tests (create sequence UTs are here, for instance)
3. Implement statement... that's the less codified thing, but it starts by adding a new case in this switch for new statements (which should fail to compile with the newly installed pgsql-ast-parser), or fix compilation errors & unit tests for other amendments to the parser.
4. Check that ALL unit tests are OK
5. Release 🎉

I'll do that soon, when I find a bit of time 😃