[Haskelldb-users] Semantics of Empty / optimization

[Michael S. <sp...@de...>]

Hi there,

I'm really a user of HaskellDB, but am trying to mine the source code
for my own work.  In reviewing the optimizations, I've stumbled upon
this code:

removeEmpty
        = foldPrimQuery (Empty, BaseTable, project, restrict, binary, group, special)
        where
          -- Messes up queries without a table, e.g. constant queries
	  -- disabled by Bjorn Bringert 2004-04-08
          --project assoc Empty   = Empty
          project assoc query   | null assoc    = Empty
                                | otherwise     = Project assoc query

I'm wondering this is true: My understanding of Empty is that it
represents a nullary relation with a single element.  (This is necessary
to it can be the identity wrt. cartesian product, I think.)  If that's
the case, this transformation is incorrect, as `project [] query' should
return as many rows as query, not just one.  Or am I overlooking
something?

-- 
Cheers =8-} Mike
Friede, Völkerverständigung und überhaupt blabla