Not the OP, but I can give two gains. First off, by passing an AST, instead of just an SQL string, we cut out a huge number of possibly SQL injection attacks. Second, in most of the projects where I've used of SQL, there's been some kind of database object that builds the actual query, which it then converts to a string. The database then takes that string and parses it into an AST. There's some performance gains to be made by skipping the middle man and just creating the AST directly.
Many years ago, I was on a project that needed to add rows to a database with a hard 10µs limit. Each rows was just four integers, so the writing part was trivial. However, allocating the string, formatting the integers to strings, then parsing the resulting string often put us over the time limit. Every time the time limit was breached, we lost about five grand. Why we were using an SQL database for this at all is a story for a different time.
