zlacker

In SQL you say something like:

  WITH RECURSIVE transitive_closure AS (
      SELECT parent, child FROM things
    UNION
      SELECT tc.parent, t.child
      FROM things t
      JOIN transitive_closure tc ON tc.child = t.parent
  )
  SELECT * FROM transitive_closure;

where `transitive_closure` is a table that gets all the results of the computation. The engine will run the second part of the query repeatedly until no new rows are added to the `transitive_closure` table. (If you change `UNION` to `UNION ALL` and there's circular paths then this will not terminate.)

Sounds a lot like whatever "tabling" is in Prolog.

>>crypto+(OP)
Likely. It depends on how the transitive_closure results are computed. In tabling it's still by resolution so you can still get stuck in infinite loops, e.g. on infinite right-recursions. I think maybe that's more similar to UNION ALL?

I should probably read a bit about this again. I rarely used recursive queries in SQL when I worked with it, not least because a couple of times I did, I got into trouble because they went haywire :)

>>YeGobl+m1
> In tabling it's still by resolution so you can still get stuck in infinite loops, e.g. on infinite right-recursions. I think maybe that's more similar to UNION ALL?

Must be. With UNION you can't end up with an infinite loop unless you have infinite data (or you're implementing a table-valued Ackermann function and so you have... not infinite results but for all practical intents, yeah).