Wrong tuple variable name in FROM clause vs. WHERE clause

[fehrenbach]

Hi,

I have a query that doesn't work:

query { for (r <-- xml)
        where (r.id == 0
           && not(empty(for (ooc <-- xml,
                             oc <-- xml)
                        where (ooc.parent == r.id && oc.parent == ooc.id)
                            [oc])))
          [r]
}

Is this sensible at all?

It produces the following SQL:

select (t1759."id") as "id",(t1759."name") as "name",(t1759."parent") as "parent",(t1759."post") as "post",(t1759."pre") as "pre"
  from xml as t1759
  where ((t1759."id") = (0)) and
     (exists (select 0 as dummy
                from xml as t1768, -- t1760 used but t1768 bound!
                     xml as t1769
               where ((t1760."parent") = (t1759."id")) and ((t1769."parent") = (t1760."id"))))

which doesn't work, because it refers to tuple variable t1760, which is not bound in any FROM clause. Instead, there is a from xml as t1768, but t1768 is never used.

Any suggestions? (I'm not entirely sure the query is valid Links code. If it's not "Your query is wrong!" is a perfectly acceptable answer...)

PS: This is using the current sessions branch at 3477fee.

[fehrenbach]

I have a Links version of the XPath evaluator from the ICFP'13 paper here:
https://github.com/fehrenbach/links-playground/blob/d5adbef68d720b05ee50d3671e1c61ed81cafc86/xpath.links
which fails for all the examples.

James ran the F# version for me, producing this for xp0:

SELECT [t1].[ID]
FROM [dbo].[Data] AS [t0], [dbo].[Data] AS [t1]
WHERE ([t0].[Parent] = @p0) AND ([t0].[Entry] = @p1) AND (EXISTS(
     SELECT NULL AS [EMPTY]
     FROM [dbo].[Data] AS [t2]
     WHERE ([t0].[Entry] = [t2].[Entry]) AND ([t0].[ID] =  
[t2].[Parent]) AND ([t2].[Entry] = [t1].[Entry]) AND ([t2].[ID] =  
[t1].[Parent])
     ))

Whereas my Links version produces this:

select (t1755."id") as "id"
  from xml as t1754,xml as t1755
 where ((t1752."parent") = (-(1))) -- t1752 is not bound, but t1754 is
   and (exists (select 0 as dummy
                  from xml as t1759
                 where ((t1752."id") = (t1759."parent")) and ((t1759."id") = (t1755."parent"))))

where t1749 is not bound and t1751 is not used.

There is some stuff about "Entry"s in the F# version, presumably it uses a slightly different schema.

[jamescheney]

Yes, the "entry" field is just an additional value to allow us to store multiple trees one table. This shouldn't matter, from the point of the apparent problem with the Links version, though.

The query in the initial comment looks like it should work to me; in general, any purely conjunctive/SQL-like query ought to work and anything that doesn't ought to yield a type error.