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
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