Polars 2.0 (Streaming Engine Default)

Proceed with caution

Why

The 5x streaming speedup, SQL-streaming default, cost-based join reordering, and native Iceberg sink metadata reuse are genuine infrastructure wins, but the same release changes the default execution engine in a way that both the official docs and a community migration write-up explain can silently reorder rows on exactly the operations most likely to feed a downstream diff or join — a correctness risk, not just a performance tradeoff — and the release is still RC, not GA.

Next

Stay pinned to Polars 1.x (or set pl.Config.set_engine_affinity('in-memory')/POLARS_ENGINE_AFFINITY) until 2.0 reaches GA; before flipping the default, grep every join/group_by/unpivot call site for order-dependence, apply the operation-appropriate maintain_order setting or an explicit sort where needed, and diff a shadow run's output against the 1.x baseline.