Locking question

Hi,

given a query like this:

select *
  from account a
 cross join lateral (
       select rate
         from exchange
        where target='USD'
          and source=a.currency
        order by date desc
        limit 1) e
 where a.id=19
   for update;

If I understand the documentation correctly, both rows, the one from
exchange and the one from account are locked, right?

In fact, I have tried it. This query blocks (currency is 'AUD' for
account #19):

select *
  from exchange
 where target='USD'
   and source='AUD'
 order by date desc
 limit 1
   for update;

However, if I create a SQL function like this:

CREATE OR REPLACE FUNCTION
exchangetousd_rate(
    cur CHAR(3),
    tm TIMESTAMP DEFAULT now()
) RETURNS TABLE(rate NUMERIC)
AS $def$

    SELECT rate
      FROM exchange
     WHERE source = $1
       AND target = 'USD'
       AND date <= $2::TIMESTAMP
  ORDER BY date DESC
     LIMIT 1

$def$ LANGUAGE sql STABLE;

and use it here:

select *
  from account a
 cross join exchangeToUSD_rate(a.currency) e
 where a.id=19
   for update;

Then the 2nd query above does not block. So, the row from the exchange
table is not locked.

Is that documented somewhere? Can I rely on it?

The plan for the last query tells me the function call is inlined. So,
in principle it's not different from the first one.

Thanks,
Torsten