1 unstable release
0.1.0 | Feb 22, 2022 |
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#1581 in Database interfaces
15KB
241 lines
pgx-named-columns
The pgx
Rust crate (github · crates · docs) is a really nice library to develop PostgreSQL extensions in Rust. Given the following Rust code:
const ALPHABET: &str = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
#[pg_extern]
fn alphabet(length: i8) -> impl Iterator<Item = (
name!(idx, i8),
name!(letter, char),
)> {
ALPHABET
.chars()
.take(length.clamp(0, 25) as usize)
.enumerate()
.map(|(i, l)| (i as i8, l))
}
...you can use the alphabet
function inside your database.
select alphabet(8);
Note how the column names are defined in Rust, line 3 and 4, using an inert declarative macro. There is currently no other way to define them. This is a problem for 2 reasons :
- Column names cannot easily be reused accross two different function that are expected to return the same value. i.e. you can't easily create an
alphabet_reverse
function that returns the exact same columns without copy-pasting code. This become a big problem when you don't have 2, but 50 columns. - It isn't clear at first glance which value of the returned tuples corresponds to which column name. If your tuple contains a lot of columns of the same type, it's incredibly easy to mix them up.
These problems could easily be solved by using a struct
as the impl Iterator
's item, but due to the way procedural macros work, they cannot access type-level information when they run. The only proper way to solve this would be a complete redesign of pgx
, which I cannot do. I opened pgx/issues#451.
Hence, the creation of this library : using filthy procedural macro hacks, including having the macro open a Rust file twice to read data outside the item it is applied to, it makes it possible to use a structure as returned rows. There are millions of way this can fail due to how badly implemented it is, but it should work for the general use-case. Here's how it looks :
const ALPHABET: &str = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
struct IndexedLetter {
idx: i8,
letter: char,
}
#[pg_extern_columns("path/to/current/file.rs")]
fn alphabet(length: i8) -> impl Iterator<Item = IndexedLetter> {
ALPHABET
.chars()
.take(length.clamp(0, 25) as usize)
.enumerate()
.map(|(idx, letter)| IndexedLetter {
idx: idx as _,
letter,
})
}
The path in the attribute parameters is probably the ugliest aspect of the macro, it is used to find the definition of IndexedLetter
.
Dependencies
~1.5MB
~37K SLoC