> For some reason, the ResultSet.next() returns false, but no exception.

Ups. I have forgetten to tell you to add "SUSPEND;" before exiting from
"selectable" procedure (a procedure that returns result set). So the code
should be:

CREATE PROCEDURE ADD_NUMBERS (NUM1 INTEGER, NUM2 INTEGER) RETURNS (RES
INTEGER) AS BEGIN
RES = NUM1 + NUM2;
SUSPEND;
END

See Language Reference for more info.

> As a side note, it appears that ResultSet fetch size is ignored, and
> the fetch size defined on Statement is used regardless. I'm setting
> ResultSet.setFetchSize(1), but the default fetch size of 400 is used
> in FBStatementFetcher.fetch().

Confirmed. Can you fill this as a bug report, so it is not forgotten before
next release?

Thanks!
Roman