Bla bla bla, subqueries in SPARQL, bla bla

Coming to you in a few days is what Jürg has been working on for last week.

Yeah, you guess it right by looking at the query below: subqueries!

This example shows you the amount of E-mails each contact has ever sent to you:

SELECT ?address
    (SELECT COUNT(?msg) AS ?msgcnt WHERE { ?msg nmo:from ?from })
    ?from a nco:Contact ;
          nco:hasEmailAddress ?address .

The usual warnings apply here: I’m way early with this announcement. It’s somewhat implemented but insanely experimental. The SPARQL spec has something for this in a draft wiki page. Due to lack of error reporting and detection it’s easy to make stuff crash or to get it to generate wrong native SQL queries.

But then again, you guys are developers. You like that!

Why are we doing this? Ah, some team at an undisclosed company was worried about performance and D-Bus overhead: They had to do a lot of small queries after doing a parent query. You know, a bunch of aggregate functions for counts, showing the last message of somebody, stuff like that.

I should probably not mention this feature yet. It’s too experimental. But so exciting!

Anyway, here’s the messy branch and here’s the reviewed stuff for bringing this feature into master.

ps. I wish I could show you guys the query that we support for that team. It’s awesome. I’ll ask around.