You may have heard of select N+1 problems before. It's the name for a class of performance problems that relate to inefficient querying of a DB. The pathological case is where you query one table for a list of items and then query another table to get the details for each item, one at a time. This is where the name comes from. Instead of the single query required, you perform N queries (one for the details of each item) and one query to get the list to begin with. Perhaps a better name would be select 1+N. The example at the end of the Latency section (earlier in this chapter) illustrates a select N+1 problem.
You will hopefully not write such bad-performing queries by hand, but an O/RM can easily output very inefficient SQL if used incorrectly. You might also use some sort of business object abstraction framework, where each object lazily loads itself from the DB. This can become a performance nightmare if you want to put a lot of these objects in a list or calculate...