Query is fast and function is slow

The query

select count(*) from documents where doc_num = '106973821' and (select
bit_or(group_access) from mda_groups where group_name in (select groname
from pg_user,pg_group where usename = 'bbob' and usesysid = any(grolist)
and (groname ~ '.*owner$' or groname = 'admin'))) & access >
'0'::bit(100);

returns very fast

If I create function

create or replace function check_for_update_permission(text,text) returns
boolean as '
declare
doc_number alias for $1;
user alias for $2;
doc_count integer;
begin

select count(*) into doc_count from documents where doc_num = doc_number
and (select bit_or(group_access) from mda_groups where group_name in
(select groname from pg_user,pg_group where usename = user and usesysid =
any(grolist) and (groname ~ ''.*owner$'' or groname = ''admin''))) &
access > ''0''::bit(100);

if doc_count > 0 then
return(true);
end if;

return(false);

end;
' language 'plpgsql';

and run "select check_for_update_permission('106973821','bbob');"
it returns the correct info but takes several minutes
Would someone please enlighten me.
Can you do something like explain analyze on a function

Thanks
Richard