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From Julian Hyde <jh...@apache.org>
Subject Re: UNION Type in Calcite
Date Thu, 17 Dec 2015 04:54:31 GMT
Jacques,

What kind of a union type were you thinking of? I was thinking of
something like a C union, where you still need to use a field to
indicate which sub-type you want. In C if you have

  typedef union {
    int i;
    double d;
  } u;

  void foo(u);
  void foo(int);
  void foo(double);

and you write

  union u;
  foo(u);

then the first "foo" gets called, and if you write

  foo(u.d);

then the last "foo" gets called.

The only difference between union and struct in C is that in the
union, the members occupy the same storage. So what I'm proposing for
Calcite is basically a struct.

Julian


On Wed, Dec 16, 2015 at 6:36 PM, Jacques Nadeau <jacques@apache.org> wrote:
> I don't think it would. We want to still do validation and we won't be
> returning a struct, we'll be returning one or the other. Think of this as a
> narrowing of the ANY type to a subset of known possibilities. Something
> that fits a particular possibility should be allowed but for example, in
> the (varbinary, varbinary[]) case, you should be able to use functions that
> only support varbinary or varbinary[] but not functions that expect int or
> varchar.
>
> Without specifically stating c.i or c.ai, you wouldn't get any validation.
> Additionally, I'd expect the validator to reject valid expressions such as
> c + 4 (where c is the union field).
>
> On Wed, Dec 16, 2015 at 4:23 PM, andrew <andrew@primer.org> wrote:
>
>> That sounds like it might fit the bill. Thanks Julien.
>>
>>
>> > On Dec 16, 2015, at 1:30 PM, Julian Hyde <jhyde.apache@gmail.com> wrote:
>> >
>> > You could declare a STRUCT(i INT, ai ARRAY(INT)) and make sure exactly
>> one of i and ai is set.
>> >
>> >> On Dec 16, 2015, at 10:51 AM, andrew <andrew@primer.org> wrote:
>> >>
>> >> I’m wondering if it would be possible to add a UNION type in Calcite.
>> >>
>> >> My use case is that I am developing a backend storage engine using
>> Calcite that only partially declares its schema. Specifically, the engine
>> declares columns such as ‘int’ when it actually means the column can
>> contain ‘int’ or ‘array of int’. There is no way to tell without actually
>> reading the table if the data is scalar or array. Indeed it can be both.
>> >>
>> >> My idea is that if Calcite had a UNION type, I could declare the
>> columns as e.g. UNION(INT, ARRAY(INT)).
>> >>
>> >> Does this sound reasonable? Or is there a better way of handling this
>> situation using the current features of Calcite?
>> >>
>> >> Thanks.
>> >> - A
>> >>
>> >>
>> >
>>
>>

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