+1 Also, in my experience, SQL query optimizers can also be unpredictable on complex queries (behavior changes with data size and statistics) which can cause them to suddenly change to slow execution plan when data grows or something is added , despite having all indexes for a good plan possible (which for interactive apps is to avoid table scans always -- they might be faster in some bad corner cases, but if data doesn't fit memory, they can't be cached and they slow down everything else).