fix(rust): Run join type coercion with correct schemas active

[wence-]

We previously ran type coercion for the left (respectively right) key expressions with the schema of the output join node active. This is incorrect since the keys may not even refer to column names in the output (if the expressions do not refer to columns). Instead, the left (right) key expressions must be coerced with the left (right) inputs active.

With this done, we now no longer need to run type coercion on the resulting Join node, and instead must just add it to the arena.

• Fixes Type coercion of inequality joins propagates wrong types to literals in expressions referencing columns of right table #19597

[wence-]

Note that this induces a behaviour change for some join expressions.

For example:

import polars as pl
left = pl.LazyFrame({"a": pl.Series([1, 2, 3], dtype=pl.Float64)})
right = pl.LazyFrame({"a": pl.Series([1, 2, 3], dtype=pl.Int64)})

q_col_keys = left.join(right, left_on=pl.col("a"), right_on=pl.col("a"))

q_col_exprs = left.join(right, left_on=pl.col("a")*2, right_on=pl.col("a")*2)

# Old behaviour
q_col_keys.collect() # => ComputeError: datatypes of join keys don't match - `a`: f64 on left does not match `a`: i64 on right

q_col_exprs.collect()

# shape: (3, 3)
# ┌─────┬─────────┬─────┐
# │ a   ┆ a_right ┆ b   │
# │ --- ┆ ---     ┆ --- │
# │ f64 ┆ i64     ┆ i8  │
# ╞═════╪═════════╪═════╡
# │ 1.0 ┆ 1       ┆ 1   │
# │ 2.0 ┆ 2       ┆ 2   │
# │ 3.0 ┆ 3       ┆ 3   │
# └─────┴─────────┴─────┘

# New behaviour

q_col_keys.collect() # => ComputeError: datatypes of join keys don't match - `a`: f64 on left does not match `a`: i64 on right

q_col_exprs.collect() # => ComputeError: datatypes of join keys don't match - `a`: f64 on left does not match `a`: i64 on right

So we're more consistent.

I think it should now also be possible to complain during schema validation, rather than compute, that the join keys have mismatching types but I couldn't spot the right place to do so.

[wence-]

Not sure the best way to test this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.92%. Comparing base (e52a598) to head (d320a3a).
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[wence-]

I think it should now also be possible to complain during schema validation, rather than compute, that the join keys have mismatching types but I couldn't spot the right place to do so.

Pushed something that does this.

[ritchie46]

Not sure the best way to test this.

Maybe test on the error raised in the example you give in this PR?

[wence-]

Not sure the best way to test this.

Maybe test on the error raised in the example you give in this PR?

Done.

[ritchie46]

Thank. Good observation that we need to resolve on the inputs. That's a tricky one.