rust - 在 polars rust 中使用 groupby 时如何避免深度复制？

Question

我有一个数据集，我需要对不同的列进行 groupby 操作。这是使用 polars 版本“0.21.1”的最小工作代码

use polars::prelude::*;
use polars_lazy::prelude::*;
use polars::df;

fn main(){
  let df = df![
    "x1" => ["a", "b", "c", "a"],
    "x2" => ["A", "A", "B", "B"],
    "y" => [1, 2, 3, 4],
    ].unwrap();

  let lf: LazyFrame = df.lazy();

  let out1 = groupby_x1(&lf);
  println!("{:?}", out1.collect());
  let out2 = groupby_x2(&lf);
  println!("{:?}", out2.collect());

}

fn  groupby_x1(lf: &LazyFrame) -> LazyFrame {
  let lf1: LazyFrame = lf.clone().groupby([col("x1")]).agg([
    col("y").sum().alias("y_sum"),
  ]);
  lf1
}

fn  groupby_x2(lf: &LazyFrame) -> LazyFrame {
  let lf1: LazyFrame = lf.clone().groupby([col("x2")]).agg([
    col("y").sum().alias("y_sum"),
  ]);
  lf1
}

但在代码中，我正在对整个 lazyframe lf 进行深度复制(使用 lf.clone()。我怎样才能避免这种情况？如果我替换 lf.clone() 在函数 groupby_x1 和 groupby_x2 中使用 lf 我得到以下错误

error[E0507]: cannot move out of `*lf` which is behind a shared reference
  --> src/main.rs:22:24
   |
22 |   let lf1: LazyFrame = lf.groupby([col("x1")]).agg([
   |                        ^^^^^^^^^^^^^^^^^^^^^^^ move occurs because `*lf` has type `polars_lazy::frame::LazyFrame`, which does not implement the `Copy` trait

error[E0507]: cannot move out of `*lf` which is behind a shared reference
  --> src/main.rs:29:24
   |
29 |   let lf1: LazyFrame = lf.groupby([col("x2")]).agg([
   |                        ^^^^^^^^^^^^^^^^^^^^^^^ move occurs because `*lf` has type `polars_lazy::frame::LazyFrame`, which does not implement the `Copy` trait

For more information about this error, try `rustc --explain E0507`.
error: could not compile `polars_try` due to 2 previous errors

Answer 1

极地 Series是Arc<Vec<ArrowRef>>附近的新类型.当你克隆一个 DataFrame只有 Arc 的引用计数递增。

换句话说，polars 从不进行深度克隆。 DataFrame 的克隆 super 便宜。