conditional aggregates

Hi list,

Can anyone advise me on creating an aggregate that would take additional
parameter as a condition ? For example, say I have a table like this

id;value
1;45
2;13
3;0
4;90

I'd like to do something like this

SELECT min_some_cond_aggregate(value,0) FROM table

to get the minimal value from table greater than 0, in this case 13.

I can't do SELECT min(value) FROM table WHERE value > 0 as this will mess my
other computations. My current solution involves a function operating on the
output of array_accum from the docs, but I'm looking for more elegant
solution.

Is this possible at all ? I'm running 8.1.

regards
mk