Re: [Haskelldb-users] Query composition bug

[Justin B. <jgb...@gm...>]

On Tue, Mar 25, 2008 at 2:08 PM, Bjorn Bringert <bj...@br...> wrote:
>  I'm thinking that it should be possible to get around this problem by
>  adding an explict cartesian product operator to use instead of >>= /
>  do. But then we are well on our way to a non-monadic interface.

How does this function look:

prod :: Query (Rel r) -> Query (Rel r)
prod (Query qs) = Query make
  where
    make (currentAlias, currentQry) =
        -- Take the query to add and run it first, using the current alias as
        -- a seed.
        let (Rel otherAlias otherScheme,(newestAlias, otherQuery)) =
qs (currentAlias,Empty)
            -- Effectively renames all columns in otherQuery to make
them unique in this
            -- query.
            assoc = zip (map (fresh newestAlias) otherScheme)
                        (map (AttrExpr . fresh otherAlias) otherScheme)
            -- Produce a query which is a cross product of the other
query and the current query.
        in (Rel newestAlias otherScheme, (newestAlias + 1, times
(Project assoc otherQuery) currentQry))

I modeled it after the binrel function. The long variable names
reflect me trying to figure out how the state (i.e. alias) was
threaded around. Continuing the example above, I can define:

cmb2x = do
  t1 <- prod tbl1
  t2 <- prod tbl2
  project $ id1 << t1 ! id1 #
              id2 << t2 ! id2

Evaluating cmb2x gives this SQL:

SELECT id13 as id1,
       id26 as id2
FROM (SELECT id25 as id26
      FROM (SELECT COUNT(id24) as id25
            FROM (SELECT id2 as id24
                  FROM sample_table as T1
                  WHERE id1 < 0) as T1) as T1) as T1,
     (SELECT id12 as id13
      FROM (SELECT COUNT(id11) as id12
            FROM (SELECT id1 as id11
                  FROM sample_table as T1
                  WHERE id1 > 0) as T1) as T1) as T2

which is far too nested. However, a one line change in mergeProject in
Optimize.hs:

safe :: Assoc -> Bool
-- merge projections that are all aggregates or no
safe assoc = not (any (isAggregate.snd) assoc) || all (isAggregate .
snd) assoc aggregates

gives SQL like this:

SELECT id13 as id1,
       id26 as id2
FROM (SELECT COUNT(id2) as id26
      FROM sample_table as T1
      WHERE id1 < 0) as T1,
     (SELECT COUNT(id1) as id13
      FROM sample_table as T1
      WHERE id1 > 0) as T2

Which is just what I want.

Justin