Aggregation interacts incorrectly with union

[tomjaguarpaw]

import Database.HaskellDB
import Database.HaskellDB.PrimQuery
import Database.HaskellDB.Query
import Database.HaskellDB.DBLayout

data Name' = Name'
instance FieldTag Name' where
  fieldName _ = "name"

data Apples = Apples
instance FieldTag Apples where
  fieldName _ = "age"

name :: Attr Name' String
name = mkAttr Name'

apples :: Attr Apples Int
apples = mkAttr Apples

applesTable :: Table (RecCons Name' (Expr String) (RecCons Apples (Expr Int) RecNil))
applesTable = Table "apples" [ ("name", AttrExpr "namecol")
                             , ("apples", AttrExpr "applecol")]

totalApples :: Query (Rel (RecCons Name' (Expr String) (RecCons Apples (Expr Int) RecNil)))
totalApples = do
  t <- table applesTable `union` table applesTable
  project (name << t!name #
           apples << _sum(t!apples))

The aggregation is done before the union. It should be after.

*Main> putStrLn $ showSql totalApples 
(SELECT namecol as name,
        SUM(age3) as age
 FROM apples as T1
 GROUP BY namecol)
UNION
(SELECT namecol as name,
        SUM(age3) as age
 FROM apples as T1
 GROUP BY namecol)

[tomjaguarpaw]

The culprit is mergeProject

https://github.com/m4dc4p/haskelldb/blob/master/src/Database/HaskellDB/Optimize.hs#L138