Optimize and improve consistency of `mapById`

Many operations retrieve multiple rows from database given multiple ids passed as input arguments. The result is then grouped by input id, using the `mapById` function. However, this function runs in `O(|results|*|inputs|)` because we iterate over the results and for each we re-iterate over the ids. A group-by operation can however be performed in `O(|results|)` only by storing into bins at each iteration of the results only.