Understanding indexes is key to demystifying database performance, and will have huge impact on one's ability to plan, contribute, and troubleshoot as one progresses in their career. Having said all that: many engineers still don’t really know what indexes ARE. Most simply know what indexes can DO. This presentation takes a look at the most commonly used index for any RDBMS: the B-tree. We will pull apart the data structure, discuss how it works, and briefly touch on some of the algorithms they use. With this baseline established, we will revisit a few common assumptions about indexes.