Beware! If you're running one-off queries, you probably won't have issues using recursion, but there are a lot of instances where you really don't want to run

Beware! If you're running one-off queries, you probably won't have issues using recursion, but there are a lot of instances where you really don't want to run a complex SQL statement for each of the 1M users reading data from your site. Selecting the proper database table structure can help dramatically! For example, take any data structured as a tree. If there are more writes than reads, it's pretty easy to put new data in place and your read becomes recursive. When you have vastly more reads than writes it's almost always better to "pre-order" the data. I've had good success with using the modified pre-ordered lists for tree structures but other table formats and processing can be used for other data types. Sometimes it's just careful but non-obvious indexing.