How about something like:
```
create trigger less_den_ten_mb_delete after insert on mqtt6 when (select sum(size_t) from mqtt6) > 1024000
begin
delete from mqtt6
where ts <= (
select ts
from (
select ts,
sum(size_t) over (order by ts desc) as size
from mqtt6
)
where size >= 1024000
limit 1
);
end;
```
and get rid of the delete trigger. You can probably do away with the when clause since if size >= 1024000 is never met then the delete will do nothing and assuming that you have an covering index on (ts, size_t) it will be very fast.