Added more stuff about AlphaSort.

Author: rxin

README.md

@@ -18,7 +18,7 @@ A list of papers essential to understanding databases and building new data syst
 ## <a name='basic-and-algo'> Basics and Algorithms
 * [The Five-Minute Rule Ten Years Later, and Other Computer Storage Rules of Thumb](http://www.cs.berkeley.edu/~rxin/db-papers/5-min-rule.pdf) (1997): This paper (and the original one proposed 10 years earlier) illustrates a quantitative formula to calculate whether a data page should be cached in memory or not. It is a delight to read Jim Gray approach to an array of related problems, e.g. how big should a page size be.
 
-* [AlphaSort: A Cache-Sensitive Parallel External Sort](http://www.cs.berkeley.edu/~rxin/db-papers/alphasort.pdf) (1995): cache-friendly sorting
+* [AlphaSort: A Cache-Sensitive Parallel External Sort](http://www.cs.berkeley.edu/~rxin/db-papers/alphasort.pdf) (1995): Sorting is one of the most essential algorithms in databases, as it is used to do joins, aggregations, and sorts. In algorithms 101 class, CS students are asked to reason about big O complexity and ignore the constant factor. In practice, however, the change in constant from L2 cache can be as big as two or three orders of magnitude. This is a good paper to learn about all the tricks fast sorting implementations use.
 
 * [Patience is a Virtue: Revisiting Merge and Sort on Modern Processors](http://research.microsoft.com/pubs/209622/patsort-sigmod14.pdf) (2014): Sorting revisited. Actually also a good survey on sorting algorithms used in practice and their trade-offs.