I am probably completely wrong, but at least it will be short ...
Could it be that
select sum(tquant)
from mtr_reg
where to_proj = $2
and pnum = $1
and (status = 'clsd' or status = 'open' or status = 'prip')
;
makes it harder for the optimizer due to the ORs, and that
select sum(tquant)
from mtr_reg
where to_proj = $2
and pnum = $1
and status in ('clsd','open','prip')
--- last line changed
;
would produce a better cost estimate?
Miguel Sofer