Simplify eval to run directly inside the Query monad#56
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shane-circuithub merged 1 commit intomasterfrom Jun 22, 2021
Merged
Simplify eval to run directly inside the Query monad#56shane-circuithub merged 1 commit intomasterfrom
eval to run directly inside the Query monad#56shane-circuithub merged 1 commit intomasterfrom
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@ocharles I'm quite confident that I have something now that will always give the expected results. |
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| rebind :: Table Expr a => a -> Query a | ||
| rebind a = Query $ \_ -> Opaleye.QueryArr $ \(_, query, tag) -> |
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Could you document what this is doing? As it occurs after laterally, it seems to kind of "undo" the need for lateral joins (as they've already served their purpose), so we go back to letting PostgreSQL do its thing.
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I don't know what you mean. This is what makes it so that if you do (\x -> (x, x)) <$> evaluate (nextval "user_id_seq"), both your xs will have the same value. This is orthogonal to laterally which adds the superfluous lateral references.
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@tomjaguarpaw at ZuriHac questioned whether the `Evaluation` monad was really unnecessary. And yes, it turns out that the `Evaluation` monad wasn't actually really adding any value. The real issue was Postgres's unspecified evaluation order (which in practice behaved like the broken `ListT` from transformers). We now maintain a stack of bindings from previous subselects in the `Query` monad, which future queries can reference. So for `evalulation`, to ensure that Postgres doesn't try to run a function once where we expect it to be run multiple times, we modify the expression to contain a bunch of superfluous lateral references to the previous queries. This ensures that it gets run every time.
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@tomjaguarpaw pointed out at ZuriHac that the
Evaluationmonad is entirely unnecessary. I've tested this change and it seems to still do what I want it to do. Not sure why I ever thought it was necessary!