Polars 2.0: why joins no longer keep their row order

7.8 relevance

Polars 2.0 streaming engine breaking change, critical for data engineering pipelines.

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Why It Matters

For data engineers and platform engineers building reliable pipelines, this silent breaking change can corrupt downstream processes that depend on positional row order, requiring explicit `maintain_order` or `sort` calls to preserve correctness.

Key Takeaways

Audit all Polars lazy joins, group_by, and unpivot operations in your pipelines and add `maintain_order` or explicit `sort` to prevent silent data corruption.

Polars 2.0: why joins no longer keep their row order

Summary

Polars 2.0, released October 6, 2026, defaults `collect()` to the streaming engine, breaking row-order guarantees for `join`, `group_by`, and `unpivot` unless `maintain_order` is explicitly set. The streaming engine processes data in parallel chunks, emitting results as they finish, which can reorder output. Out-of-core processing is also enabled by default, spilling to disk at ~80% RAM usage with a 64 GB budget, though out-of-core joins and group_by remain on the roadmap.

Author

Nikoloz Turazashvili (@axrisi)