How often do you think about the reasons why your favorite RDBMS sucks? Last few months I was doing this quite often and yes, my favorite RDBMS is MySQL. The reason why I was thinking so because one of my recent tasks at Scribd was fixing scalability problems in documents browsing.
The problem with browsing was pretty simple to describe and as hard to fix – we have large data set which consists of a few tables with many fields with really bad selectivity (flag fields like is_deleted, is_private, etc; file_type, language_id , category_id and others). As the result of this situation it becomes really hard (if possible at all) to display documents lists like “most popular 1-10 pages PDF documents in Italian language from the category “Business” (of course, non-deleted, non-private, etc). If you’ll try to create appropriate indexes for each possible filters combination, you’ll end up having tens or hundreds of indexes and every INSERT query in your tables will take ages.