This is very true. I created something like this for our database engine, ElevateDB, in 2011 or so. It divides up the bitmap space for a signed, 32-bit intege

This is very true. I created something like this for our database engine, ElevateDB, in 2011 or so. It divides up the bitmap space for a signed, 32-bit integer (our engines are embedded and have a limit of High(Int) on the number of rows) into nodes and, optionally, has a parallel array in each node for attaching object instances to each bit. Originally I just used them for representing sets of rows, but once you start using sparse bitmaps like this, you realize that you can use them for all sorts of things. Now I use them in buffer managers and many other areas. They're extremely efficient at locating a given row/tuple by its integer ID.