When I run the SQL below, I get an error on the third call to foo() : "ERROR: could not open relation with OID 884693". I'm quite sure this OID belongs to the TOAST table corresponding to the temporary table created by foo() during the third call. The fourth call works fine again.
I suspect the third one fails because the data is too large (even when compressed) to be stored without using toast. I can't reproduce the issue if for example I use a long string of identical characters instead of "random" ones. My guess would be that the TOAST table is still somehow referenced by the result value, even though the table itself has been dropped by the time the result value is used.
I have tested this myself on 8.4.4 and it has also been verified on a 9.1 by someone on #postgresql.
CREATE OR REPLACE FUNCTION foo(size_ INTEGER) RETURNS TEXT AS $EOF$
acc_ TEXT := '';
EXECUTE 'CREATE TEMPORARY TABLE foo_tab(blob TEXT NOT NULL)';
-- Construct a string with random characters to prevent compression (with high probability)
EXIT WHEN length(acc_) >= size_;
acc_ := acc_ || chr(ceil(random()*64)::integer + 32);
EXECUTE 'INSERT INTO foo_tab(blob) values (' || quote_literal(acc_) || ')';
EXECUTE 'SELECT * FROM foo_tab LIMIT 1' INTO cur_rec_;
EXECUTE 'DROP TABLE foo_tab';
$EOF$ LANGUAGE plpgsql;
SELECT md5(foo(40000)); -- This one breaks on my 8.4.4
SELECT md5(foo(30)); -- And this one works fine again
DROP FUNCTION foo(INTEGER);
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