Function execute_plan 

pub fn execute_plan(
    cache: &TableCache,
    table_name: &str,
    plan: LogicalPlan,
) -> ExecutionResult<Vec<Rc<Row>>>

Executes a logical plan using the query engine.

This function:

1. Builds execution context with index information
2. Creates QueryPlanner with unified optimization pipeline
3. Plans and optimizes the query (logical + physical)
4. Executes using PhysicalPlanRunner