Обсуждение: type aliases

I can create an alias of a type like:

CREATE DOMAIN myvarchar varchar;

But I can't declare a myvarchar with a variable size, eg "mycolumn myvarchar(42)" and from what I've read this is the
wayit DOMAIN is supposed to work so I can't do it that way. 

Is there a way to define myvarchar the way I want to? Can I use CREATE TYPE and just mirror the declaration of the
underlyingtype? 

Thanks

James

>
> You probably should define your domain like this:
>
>    CREATE DOMAIN myvarchar varchar(42);
>

That's what I thought, so it won't do what I want. I need to be able to set the length at the time of declaration.

So suppose I wanted to implement myvarchar in C. In my _in function, how do I know how big my column declaration is? Eg
ifsomeone tries to insert 50 characters into my 42 character field, how do I get the declared length and then tell
postgresthat the data to be inserted is too big? 

Thanks

James

James Harper wrote
>>
>> You probably should define your domain like this:
>>
>>    CREATE DOMAIN myvarchar varchar(42);
>>
>
> That's what I thought, so it won't do what I want. I need to be able to
> set the length at the time of declaration.
>
> So suppose I wanted to implement myvarchar in C. In my _in function, how
> do I know how big my column declaration is? Eg if someone tries to insert
> 50 characters into my 42 character field, how do I get the declared length
> and then tell postgres that the data to be inserted is too big?
>
> Thanks
>
> James

IMO. You are abusing the type system to implement things that should be
defined using CHECK constraints.  If indeed you are working with string I
would avoid length-limited type mods and rely on checks/constraints.  There
isn't any viable alternative for numeric scale/precision type modifiers
though :(

There is no current way for a function body to examine the type modifiers of
its input, domain or otherwise.

David J.




--
View this message in context: http://postgresql.1045698.n5.nabble.com/type-aliases-tp5792148p5792207.html
Sent from the PostgreSQL - general mailing list archive at Nabble.com.

David Johnston <polobo@yahoo.com> writes:
> James Harper wrote
>> So suppose I wanted to implement myvarchar in C. In my _in function, how
>> do I know how big my column declaration is? Eg if someone tries to insert
>> 50 characters into my 42 character field, how do I get the declared length
>> and then tell postgres that the data to be inserted is too big?

> There is no current way for a function body to examine the type modifiers of
> its input, domain or otherwise.

Actually the point here is to know the required type modifier of the
*output*.  Type input functions do get passed the target typmod, as a
separate argument.  See varcharin() for an example.

            regards, tom lane