Re: Imperative Query Languages

On Wed, Jul 5, 2017 at 8:42 AM, Jason Dusek <jason(dot)dusek(at)gmail(dot)com> wrote:

>
>
> If we imagine network databases have one layer:
>
> Imperative Plan
>
> And SQL databases have two:
>
> Declarative Query -> Imperative Plan
>
> It seems reasonable to say, LINQ, &al. have three:
>

> Imperative Syntax -> Declarative Query -> Imperative Plan
>
> Fortress is rather the same, since it translates imperative to functional
> to assembly.
>

I am curious where you see LINQ as starting at an imperative syntax.

Here's a good case that illustrates the problem I think. Suppose the
following is understood imperatively:

FOR x IN RANGE student
SELECT WHERE x.age < 25
PROJECT ALL(x), lock_if_possible(x.id)

Now, lock_if_possible has side effects. If we understand this to be
imperative, then we have no possibility of turning this into a declarative
query because we are interested in the side effects. So you cannot say
that this is equivalent to the SQL of

SELECT *, lock_if_possible(id)
FROM student
WHERE age < 25

The reason is that while the imperative version represents *one* valid
interpretation of the declarative, there are other interpretations of the
declarative that are not at all equivalent. The hoops we have to jump
through to make this work in an imperative way in SQL are sometimes rather
amusing.

>
> Kind Regards,
> Jason
>

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