Caching runtime plan data that is determined not to change for different invocations of a query plan. In some embodiments, a computing system accesses information that specifies a query plan generated for a first database query and generates a first runtime plan for the first database query based on the query plan. In some embodiments, the system caches information generated for the first runtime plan that is determined not to change for different invocations of the query plan. For example, transformation code may include separate functions for mutable and immutable state. In some embodiments, the system retrieves and uses the cached information to generate a second runtime plan for a second database query. Disclosed techniques may improve performance of query plan transformations that hit in the runtime plan cache.