What you picked are the happy paths. Here's the common scenario: deals = sql("select * from current_deals ..."); // Show the user the deals. // User tak

What you picked are the happy paths. Here's the common scenario: deals = sql("select * from current_deals ..."); // Show the user the deals. // User takes his sweet time to ponder over the deals. // Meanwhile, the deals are gone. order = new_order() order.deal_id = deals[5].id order.insert() // User calls in to complain never receiving the deal. // Ok, need to verify the deal still exists. sql("select deal from current_deals where id = ? for update", deals[5].id) ... insert ... sql("commit") That's second select..for update is referential checking in code.