Re: Question about casts

Martijn van Oosterhout wrote:
> On Thu, May 18, 2006 at 05:41:14PM +0200, Thomas Hallgren wrote:
>   
>> Just out of curiosity (and most likely, ignorance). Why can't I cast an 
>> array of strings into a string? I.e.
>>
>>  thhal=# select ('{"a","b"}'::varchar[])::varchar;
>>  ERROR:  cannot cast type character varying[] to character varying
>>     
>
> Why would you need to? What would you expect to happen? Joined with a
> seperator, no seperator, with parenthesis?
>
>   
Well, let's assume I use JDBC. I write code like:
 ResultSet rs = stmt.executeQuery("SELECT arrValue ..."); while(rs.next())     String v = rs.getString(1);

The tuples received by the result set contains String[]. If I let 
PL/Java convert it (I don't currently), it will be according to Java 
semantics. I'd like to convert it using PostgreSQL semantics instead. So 
I change my statement to:
 "SELECT array_out(arrValue) ..."

that works of course. What baffles me is that I cannot write
 "SELECT arrValue::varchar"


> What's the benefit of a cast over a function call?
>
>   
None whatsoever. But PostgreSQL enables a lot of casts for some reason 
or another right? Why not this one?

> Every cast costs space and lookup time. Any user can add their own
> casts if they want, but the system generally only includes the ones
> useful to many people or those required for standards complience.
>
>   
OK. I can live with that. I would have thought that casting into the 
string types was something that could be hardwired since the backing 
functions are mandatory.

Regards,
Thomas Hallgren