Function execute_query 

pub fn execute_query<S: KVStore, C: Catalog>(
    txn: &mut SqlTransaction<'_, S>,
    catalog: &C,
    plan: LogicalPlan,
) -> Result<ExecutionResult>

Execute a SELECT logical plan and return a query result.

This function uses an iterator-based execution model that processes rows through a pipeline of operators. This approach:

• Enables early termination for LIMIT queries
• Provides streaming execution after the initial scan
• Allows composable query operators

Note: The Scan stage reads all matching rows into memory, but subsequent operators (Filter, Sort, Limit) process rows through an iterator pipeline. Sort operations additionally require materializing all input rows.