I found that selecting a set of Detail (TransactionItems) records using a nested select of the the Master (Transactions) Id (TransactionNo), was very slow. Why is this? I would have thought that
it would have used the TransactionNo index to rapidly locate the few TransactionItem records associated with each Transaction.

For example

Select * from TransactionItems TI
where TI.TransactionNo in (Select TransactionNo from Transactions
where PersonNo = 200)

TransactionNo is an indexed field of TransactionItems

Is this the right way to approach this?