I enjoy ppx_rapper as mentioned quite a bit.
Another option with what looks like a quality ppx facility for easy syntactic integration into your codebase is pgocaml, which takes a somewhat different approach to land in a similar spot w.r.t. compile-time checking of queries and such. (/ht @glennsl for pointing towards it @ [ANN] First release of PGX (Pure-OCaml PostgreSQL client) with some good discussion of pgocaml’s benefits).