Macro duck_projection 

macro_rules! duck_projection {
    () => { ... };
}

Returns the 0-based column indices the current query actually needs, as a Vec<usize>, from inside a #[duckdb_table_function(projection_pushdown)] function body.

Empty if the query didn’t request pushdown (every column is wanted).

Panics

Panics if called outside a table function’s init callback — contained by this crate’s callback machinery, so it always surfaces as a normal query error, never undefined behavior.

Examples

use better_duck_core::{connection::Connection, duck_projection, duckdb_table_function};

#[duckdb_table_function(columns("a", "b"), projection_pushdown)]
fn two_cols() -> impl Iterator<Item = (i32, i32)> + Send {
    let wanted = duck_projection!();
    // `wanted` lists which of columns 0 (`a`) / 1 (`b`) the query needs.
    let _ = wanted;
    std::iter::once((1, 2))
}
let mut conn = Connection::open_in_memory()?;
two_cols::register(&mut conn)?;