Can you tell what the Catalyst optimizer in Spark is?

The Catalyst optimizer is Spark SQL’s query optimization framework. It takes a logical query plan and applies a series of transformation rules to produce an optimized logical plan. These rules perform tasks such as predicate pushdown, constant folding, and join reordering. After logical optimization, Catalyst generates a physical plan by selecting the best execution strategy (e.g., sort‑merge join vs. broadcast join) based on cost estimates. The optimizer also handles type coercion, column pruning, and code generation for efficient execution. By abstracting query logic from execution details, Catalyst enables Spark to run SQL, DataFrame, and Dataset queries efficiently across a cluster.