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...
