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It is very non-obvious how to apply limit/offset/order conditionally #491

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neongreen opened this issue Jul 7, 2020 · 7 comments · May be fixed by #633
Open

It is very non-obvious how to apply limit/offset/order conditionally #491

neongreen opened this issue Jul 7, 2020 · 7 comments · May be fixed by #633
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docs enhancement

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@neongreen
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neongreen commented Jul 7, 2020 •
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I want to apply limit_ conditionally, like this:

select_ $
  if limitNeeded then limit_ 10 else id $
  all_ someTable

This fails with cannot construct the infinite type: s ~ QNested s. I get that limit_ changes the type and id doesn't, but I can't figure out what I can use instead of id. I thought subselect_ could work, but I'm getting ugly nested queries when limitNeeded is false:

SELECT `t0`.`res0` AS `res0`, `t0`.`res1` AS `res1` FROM
  (SELECT `t0`.`email` AS `res0`, `t0`.`enabled` AS `res1` FROM `table` AS `t0`)

Any advice?
@neongreen
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neongreen commented Jul 7, 2020 •
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I can use unnest instead of id, but this seems risky:

unnest ::
  Q MySQL db (QNested s) (table (QExpr MySQL (QNested s))) ->
  Q MySQL db s (table (QExpr MySQL s))
unnest = unsafeCoerce

@neongreen
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neongreen commented Jul 7, 2020 •
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Oh okay, polymorphism saves the day! As long as I'm taking something that is forall s., I'm good.

  let applyLimit ::
        (forall s. Q MySQL db s (table (QExpr MySQL s))) ->
        (forall s. Q MySQL db s (table (QExpr MySQL s)))
      applyLimit x = case mbLimit of
        Nothing -> x
        Just n -> limit_ (toInteger n) x

It's important not to use maybe or fromMaybe here though.

@3noch
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3noch commented Jul 7, 2020

Yuck. We should document this...

@3noch
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3noch commented Jul 7, 2020

Thanks for reporting.

@3noch
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3noch commented Jul 7, 2020

Maybe it really is worth having a noLimit_ function just to make this discoverable.

@neongreen neongreen reopened this Jul 7, 2020
@neongreen
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I'll reopen then.

@neongreen neongreen changed the title How to apply limit/offset/order conditionally? It is very unobvious how to apply limit/offset/order conditionally Jul 7, 2020
@neongreen neongreen changed the title It is very unobvious how to apply limit/offset/order conditionally It is very non-obvious how to apply limit/offset/order conditionally Jul 7, 2020
@thomasjm
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thomasjm commented Jan 9, 2021

Is there a way to write the applyLimit workaround above so that it works on an arbitrary return value? (I.e. not just some kind of table.) I'm having trouble writing such a function due to the QNested s stuff in limit_'s signature.

I'd be happy to open a PR for a noLimit_ if someone could give hints on how it would be implemented.

@thomasjm thomasjm linked a pull request Feb 18, 2022 that will close this issue
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Add limitMaybe_, offsetMaybe_ + tests thomasjm/beam
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@3noch @kmicklas @neongreen @thomasjm