According to the documentation, you can't index on XML directly because there are no XML comparison operators (I suspect this is to avoid hair-pulling debates o

According to the documentation, you can't index on XML directly because there are no XML comparison operators (I suspect this is to avoid hair-pulling debates over whether `<foo x='y'>` should compare equal to `<foo x="y"></foo>`). But you can serialize the XML, or a fragment of XML retrieved by XPath, and cast it to text, and you can build an index on the result of that serialization. (PostgreSQL has supported indexing on expressions for a loong time.) I think the place you lose is if you have some arbitrary XPath expression for which you haven't constructed a DB index yet, and you want to search on that expression.