@Olaf You mean that formatting a Scientific number as String is more efficient than formatting an Integer/Double in its usual representation? How about parsing?

I'm not going to defend the 'efficiency' of Haskell's String formatting/parsing, but ... What is your evidence that number/String is a concern here? It's all happening in-memory. Whereas sending whatever to SQL then needs putting it to disk, which is orders-of-magnitude slower. (Or is this an in-memory database?) This thread has already devoted more cycles to speculation than you're going to reclaim through number formatting, methinks.