To add to Tom's statement: The data types are quite important. In case you have numbers to index on, don't use "numeric" as data type, unless you really have to. We found out in one of our applications that a search over integer is much more efficient.
Be as well aware, the postgresql supports different types of indices. I have seen a couple of benchmarks in german it-newspapers comparing mysql and postgresql showing that for large data-sets postgres actually is superior.
