SelectParser: add support for grouping labels

[radu-gheorghe]

The Select (JSON) syntax didn't support a way to change the label of grouping results. The YQL variant does with something like:

each(...) as(mylabel)

This commit fixes this gap. Something like this now works:

{
  "all": {
    "group": "explanation_attribute",
    "each": {
      "output": "count()",
      "label": "explanations"
    }
  }
}

Note how the label is within each. Feel free to disagree with that :) It's in the name of simplicity/reliability, because JSON grouping is translated to "normal" grouping, where as needs to go after each (IIUC). The current implementation guarantees that each is there. If we did it one level higher (which is more natural), we'd have to do more validation to make sure we don't put as in the wrong place.

Speaking of validation, we do a reasonable job (e.g., reject the request if label is upper in the hierarchy - see tests for more), but we don't cover everything. AFAIK, the only corner case is when someone has an array for each (which is unlikely, IMO):

    "each": [{
      "output": "count()",
      "label": "explanations"
    }]

In this case, we silently drop the label instead of rejecting the request as invalid. I think this is in line with the rather loose validation that we do for JSON grouping in general: we rely on the underlying grouping syntax parser to catch some of the errors. You can argue that's not nice, but refactoring for better JSON validation is a separate task, IMO.

[boeker]

Seems reasonable to me 🙂

[johansolbakken]

Looks good to me 👍🏻 , but should we run it by the architecture meeting as well?

[radu-gheorghe]

Ah, that would be a good idea 🙈 I'll do it later.