I'm building a CRUD application that pulls data using Persistent and executes a number of fairly complicated queries, for instance using window functions. Since these aren't supported by either Persistent or Esqueleto, I need to use raw sql.
A good example is that I want to select rows in which the value does not deviate strongly from the previous value, so in pseudo-sql the condition is WHERE val - lag(val) <= x. I need to run this selection in SQL, rather than pulling all data and then filtering in Haskell, because otherwise I'd have way to much data to handle.
These queries return many columns. However, the RawSql instance maxes out at tuples with 8 elements. So now I am writing additional functions from9, to9, from10, to10 and so on. And after that, all these are converted using functions with type (Single a, Single b, ...) -> DesiredType. Even though this could be shortened using code generation, the approach is simply hacky and clearly doesn't feel like good Haskell. This concerns me because I think most of my queries will require rawSql.
Do you have suggestions on how to improve this? Currently, my main thought is to un-normalize the database and duplicate data, e.g. by including the lagged value as column, so that I can query the data with Esqueleto.