random record from small set

I am trying to retrieve a random record (according to a chance
attribute) from a small set of records, each with a "chance" attribute.
This may eventually be somwhat of a performance concern, so I'd like to
make sure I'm doing this right.

Here's what I have so far:

create table r1 (
   i int,
   chance numeric
)
create or replace function randrec() returns int as $$
    $res = spi_exec_query('select i,chance from r1');
    $r = rand;
    $accum = 0;
    $i = 0;
    while($accum < $r) {
        $accum += $res->{rows}[$i++]->{chance}
    }
    return $res->{rows}[$i-1]->{i};
$$ language plperl;

test=# select * from r1;
 i | chance
---+--------
 1 |   0.25
 2 |   0.20
 3 |   0.15
 4 |   0.10
 5 |   0.30


That seems to work, in that out of 10k times, I got the following
numbers of each:
1 2479
2 1959
3 1522
4 950
5 3090

But I have a few questions:
* Am I right to use NUMERIC for the chance attribute?
* Does perl's arithmetic leave me with the chance that those numeric
values don't add up to 1.00 (and in this case that could mean an
infinite loop)?
* In my design I'll need a constraint trigger making sure that the
numbers add up to 1.00. Will that be a performance problem for
operations on the table that don't modify the chance attribute?
* Is there a better way?
* Does spi_exec_query pull the entire result set into memory at once? Is
there a point at which performance could be a serious problem if there
are a large number of items to select among?

Regards,
    Jeff Davis