I had an application where being able to write a customized aggregation function (like SUM, MAX, etc. but your own function) would have been a huge help, but un

I had an application where being able to write a customized aggregation function (like SUM, MAX, etc. but your own function) would have been a huge help, but unfortunately the database was an RDS MySQL instance where that is impossible. MySQL requires those functions to be written in C, which RDS doesn't allow (you can do it with self-hosted MySQL of course). There was once a version of MariaDB that allowed writing those functions in Javascript, which would have been fine, but I don't think it was ever released. Being able to write ordinary mathematical functions (UDF's, user defined functions) can also be a big help. I remember doing some completely crazy stuff to do those entirely with MySQL builtins, because again, loading C functions wasn't permitted in RDS.