My first master's research topic was about spatial indexes. I learned about R-Trees and R+-tree, R*-trees and other various but very similar topics. This may not be published information, but what spatial index type does ESRI use? As a secondary question can this type be changed and how? There are some situations where a different type of index would boost performance.
In Oracle's case it can use the ST_Geometry spatial data type which can be indexed to either an R-tree or a quad tree or both: http://docs.oracle.com/html/A88805_01/sdo_intr.htm
Other databases use binary storage mechanisms which are non-spatial by default and do not seem to use the spatial indexing available. In SQL 2005 and lower spatial datatype didn't exist, but ESRI was still able to use SQL databases. According to this case study SQL 2012: http://www.microsoft.com/casestudies/Case_Study_Detail.aspx?CaseStudyID=710000000547 it uses the automated spatial performance type, but doesn't list what kind of R-Tree it is.
Bonus points for reading material answering this question.