Having done a lot of SQL optimisation stuff I have doubts that this is possible. The problem is that it is much easier to go from a declarative to an imperative plan than it is to go the other way. In fact sometimes we use SQL the way your first code works and then it is often a problem.
For example, consider the difference between an EXISTS and an IN query, or between an INNER JOIN and a LATERAL JOIN. PostgreSQL's optimiser is amazing at identifying cases where these are equivalent and planning accordingly, but it is extremely easy to get just outside the envelope where the optimiser gives up and has to default back to an imperative interpretation of these. Proving that two imperative approaches are equivalent is a lot harder than proving that two different imperative approaches implement the same declarative request. In other words, going imperative -> declarative strikes me as a far, far harder problem than the other way.
Also I have done a little bit of work on Apache Spark and there it is extremely important to understand the imperative side of the data flow in that case (what is partitioned and what is not).
I can not argue these points with you; but Fortress is a good example of imperative looking code that translates to a functional/declarative core; as indeed is monadic or applicative code. LINQ is a more recent and widespread example -- though not encompassing an entire language -- of something that has an imperative form while being declarative under the hood. Scala's for comprehensions -- more or less monad comprehensions --are another.
With regards to Spark, I assume for comprehensions are an important part of the interface?
