[PERFORMANCE] slow small delete on large table

A 7.3.4 question...

I want to "expire" some data after 90 days, but not delete too
much at once so as not to overwhelm a system with precariously
balanced disk I/O and on a table with millions of rows. If I
could say it the way I think for a simple example, it'd be
like this:

delete from mytable
where posteddatetime < now() - '90 days'
limit 100;

Of course, that's not legal 7.3.4 syntax. These are both too
slow due to sequential scan of table:

delete from mytable where key in (
select key
from mytable
where posteddatetime < now() - '90 days'
limit 100);
or
delete from mytable where exists (
select m.key
from mytable m
where m.key = mytable.key
and m.posteddatetime < now() - '90 days'
limit 100);

Tried to use a cursor, but couldn't figure out the syntax
for select-for-delete yet, or find appropriate example on
google. Any clues?

TIA.