It looks like gmax nailed your specific question, so I'll toss in a bit of general advice. For playing in the SQL optimization sandbox, I've found Gulutzan and Pelzer's SQL Performance Tuning to be quite useful. It discusses the general theory of how RDBMSs work under the covers (including details on cost-based query optimization and the performance characteristics of various table implementations), then contrasts how 8 different RDMBSes (including MySQL, Oracle, and SQL Server) respond to various query adjustments and optimizations. It's great fodder for reasoning through performance problems.