XML vs SQL – Which Is Better for Small Projects?

I've done a similar thing where I serialize object data to XML for storage. This removes the burden of having tables, columns, and relationships in place to hold data for complex objects. The process can be helped by using a schema that the result XML conforms to, and the database tables can be used for meta information about the objects in question. In my case, expanding database schema to accommodate the object layout would be a big task!

One could go and make wild speculations here, or even educated guesses, but the truth is that it probably doesn't matter, and if it does, it's going to depend on a lot of other factors. The only way to find out is to profile, and when you do, first be sure to rule out other factors (system load, fragmentation, etc.).

My bet would be that the difference is going to be negligible, and that there's much more to win elsewhere - set up sensible indexes, profile joins vs. subqueries, use caching where possible, experiment with many small queries vs. few large queries, avoid unnecessary trips to the database, buffer accumulated results, etc.

The thing is, an integer vs. bool optimization will never give you more than a linear speed increase, and the increase won't be large - if you get 5%, consider yourself lucky. Those 5% will get smashed if your query does an outer join requiring full scans on both joined tables when one table is really large and you add one row to the other.