Polars 2.0 RC makes the streaming engine the default and drops row-order guarantees.

Announced September 2, every LazyFrame query now runs on the streaming engine by default, which the team claims is roughly 5x faster with substantially lower memory on typical workloads. The breaking change with teeth: streaming no longer guarantees row ordering for joins and group_by, so callers depending on it must pass maintain_order=True or fall back to the in-memory engine. is_in now rejects lossy Int64-to-Float64 coercion and horizontal concat requires matching heights. pip install polars==2.0rc1.