Optimize <|>: implementation of Tabulation
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ocharles
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Jun 9, 2022
`Tabulation` has an instance of `AltTable`, which is intended to uphold the following law: ```haskell fromQuery a <|>: fromQuery b = fromQuery (a <|>: b) ``` The previous implementation was not actually defined in terms of `Query`'s `<|>:` (i.e., `UNION ALL`), because not every `Tabulation` can be safely `toQuery`'d. Instead it used a combination of `alignWith`, `catNonEmptyTable` and `some` that worked even on "magic" `Tabulation`s. However, using `unsafePeekQuery`, we can actually "statically" determine if a `Tabulation` is "magic" or not, which means we can selectively switch the implementation to use `Query`'s `<|>:` where possible. This produces a simpler and usually faster query.
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Tabulationhas an instance ofAltTable, which is intended to uphold the following law:The previous implementation was not actually defined in terms of
Query's<|>:(i.e.,UNION ALL), because not everyTabulationcan be safelytoQuery'd. Instead it used a combination ofalignWith,catNonEmptyTableandsomethat worked even on "magic"Tabulations.However, using
unsafePeekQuery, we can actually "statically" determine if aTabulationis "magic" or not, which means we can selectively switch the implementation to useQuery's<|>:where possible. This produces a simpler and usually faster query.