ReuseSubquery is a physical query optimization (aka physical query preparation rule or simply preparation rule) that QueryExecution uses to optimize the physical plan of a structured query by FIXME.

Technically, ReuseSubquery is just a Catalyst rule for transforming physical query plans, i.e. Rule[SparkPlan].

ReuseSubquery is part of preparations batch of physical query plan rules and is executed when QueryExecution is requested for the optimized physical query plan (i.e. in executedPlan phase of a query execution).