Re: Composite datatypes, dynamic member fields

Robert Staudinger <robson(at)stereolyzer(dot)net> writes:
> One idea is to implement a . operator on a basic data type and return
> the value
> for the corresponding field from the "operator function".
> E.g.
> "select * from mytable where mytype.mymember='x'"
> could call something like
> mytype_member_read( mytype, member_name )
> but I'm not sure which datatype member_name would be in this case.

PG has always had the ability to define functions that could be
notationally treated as fields. A trivial example:

test72=# create table tours(depart date, return date);
CREATE
test72=# insert into tours values('2002-01-01', '2002-01-10');
INSERT 525275 1
test72=# insert into tours values('2001-12-15', '2002-01-05');
INSERT 525276 1
test72=# create function numdays(tours) returns int as '
test72'# select $1.return - $1.depart' language sql;
CREATE
test72=# select *, tours.numdays from tours;
depart | return | numdays
------------+------------+---------
2002-01-01 | 2002-01-10 | 9
2001-12-15 | 2002-01-05 | 21
(2 rows)

The computed field doesn't quite have the same status as real fields
--- notice that * doesn't know about it in the above example --- but
it's a useful technique anyway.

regards, tom lane