What is Adaptive Query Execution (AQE) in Spark and why does it matter?

AQE dynamically optimizes Spark query plans at runtime based on actual data statistics.

Adaptive Query Execution (AQE) in Apache Spark allows the engine to modify an execution plan after it has started running. Traditional Spark optimization relies on static estimates, which are often inaccurate. AQE uses runtime metrics such as shuffle partition sizes, data skew detection, and join statistics to adjust execution dynamically. It can switch join strategies (broadcast vs shuffle join), coalesce partitions, and mitigate skew automatically. This significantly improves performance in unpredictable workloads where data distribution is not known in advance.