What does Rebind do in `optionalRestrict`?

[stevemao]

I'm trying to refactor all my leftJoins to use optionalRestrict based on the docs

It compiles well but the query behaves differently.

when right query returns nothing, and left query returns something, leftJoin would still return rows with fields that come from right query null. But optionalRestrict would just return empty rows.

In the generated SQL, I see a rebind always true in the right table. Removing that and in the where it works as expected. I can't figure out what rebind does here. I believe simple code example like the one in the docs can replicate the problem but let me know if not. Thanks.

[tomjaguarpaw]

That sounds strange. Did you replace your left join using this pattern? When there are missing rows in the right query you are supposed to get nothingFields in the right of the tuple. Is that not what you're getting?

proc () -> do
  fieldsL <- qL -< ()
  maybeFieldsR <- optionalRestrict qR -< curry cond fieldsL
  returnA -< (fieldsL, maybeFieldsR)

[tomjaguarpaw]

Here's an example. The output is

*Main> run
[((1,10),Nothing),((2,20),Just (2,"Hello"))]
Right ()

{-# LANGUAGE Arrows #-}

module Main where

import Control.Arrow (returnA)

import Opaleye
import qualified Database.Postgres.Temp as T
import qualified Database.PostgreSQL.Simple as S
import qualified Database.PostgreSQL.Simple.Options as O
import GHC.IO.Exception (ExitCode)

qL = values [(1, 10), (2, 20)]

qR = values [(2, toFields "Hello"), (3, toFields "Good bye")]

exampleJoin :: Select ((Field SqlInt4, Field SqlInt4),
               MaybeFields (Field SqlInt4, Field SqlText))
exampleJoin = proc () -> do
  fieldsL@(x, t) <- qL -< ()
  maybeFieldsR <- optionalRestrict qR -< (\(y, s) -> x .== y)
  returnA -< (fieldsL, maybeFieldsR)

run = withTempDBConnection $ \conn ->
  print =<< runSelectI conn exampleJoin

withTempDBConnection :: (S.Connection -> IO a)
                     -> IO (Either T.StartError a)
withTempDBConnection f = T.with $ \tempDB ->
  f =<< S.connectPostgreSQL (T.toConnectionString tempDB)

[stevemao]

Sorry @tomjaguarpaw , it's not optionalRestrict causes my problem. I get maybeFieldsR and then I need to aggregate on it. I use catMaybeFields to get rid of the maybe but it's actually wrong and that causes my problem. I created another issue for that https://github.com/tomjaguarpaw/haskell-opaleye/issues/547