Trouble with null text fields

Using Postgres 6.5 beta (snap 12 apr), on Linux i386.  I moved up from
6.4.2 when I couldn't get things working.

I'm having difficulty dealing with null text/varchar fields.  I need
to be able to interpret null values as the null string '' for the
purposes on concatenation.


1) ----
For example, the query:
   SELECT (field_one || field_two) from t1;

Will return the concatenation of the two fields.  If either of the fields
is null, it is interpreted as the empty string '' and the correct answer
is printed.  But:
   SELECT * from t1 where ( field_one || field_two = 'something' )

This does NOT work is either field_one or field_two is null.  Same result
with the textcat() function.


2) ----
Next I tried using "case", getting a parse error at or near "then":
   SELECT ( case field_one when null then '' else field_one end ) from t1;

This one gave me "ERROR: copyObject: don't know how to copy 704":
   SELECT ( case field_one when 'string' then 'other' else 'third' end)
from t1;



3) ---
I tried writing a function that takes a "text" type and returns '' is the
string
is null.  Never could successfully do a null test on a function parameter.


4) ----
I also tried writing my own concat function, and found that passing
null fields into a user function doesn't seem to work either.  So I tried
passing the whole thing in as a TUPLE, since you can determine whether a
field is null with the GetAttributeByName() call.

The creation:
   CREATE FUNCTION mycat(text,text) returns text as '/usr/.../file.so'
langauge 'sql';

This worked great, even with null values:
   SELECT mycat(field_one, field_two) from t1;

This crashed the backend, with a "Memory exhauted in AllocSetAlloc()" error:
   SELECT * from t1 where mycat(field_one, field_two) = 'something';

So I tried making an index, and got:  "DefineIndex(): Attibute t1 not found"
   CREATE INDEX t1_ix on t1 ( mycat(t1) text_ops );


Any help is appreciated!! Sorry for the novel!!  -glenn