Re: BUG #17376: Adding unique column with a function() default results in "could not read block 0 in file" error

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От David G. Johnston
Тема Re: BUG #17376: Adding unique column with a function() default results in "could not read block 0 in file" error
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Msg-id CAKFQuwYmmCdUo1KqLeZFJ0i+siLugpv-hT+yD_by=zoT-LkH4Q@mail.gmail.com
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Ответ на BUG #17376: Adding unique column with a function() default results in "could not read block 0 in file" error  (PG Bug reporting form <noreply@postgresql.org>)
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On Fri, Jan 21, 2022 at 4:20 AM PG Bug reporting form <noreply@postgresql.org> wrote:

Bug reference:      17376

CREATE FUNCTION myfunc() RETURNS TEXT LANGUAGE plpgsql AS $$
  BEGIN
    SELECT r FROM t1;
    RETURN random()::text;
END $$;

ALTER TABLE t1 ADD COLUMN r TEXT NOT NULL UNIQUE DEFAULT myfunc();
--

Results in the error:
ERROR:  could not read block 0 in file "base/84505/84705": read only 0 of
8192 bytes

The error message content returned is what I suspect of being a bug, not so
much that this SQL didn't work.


About the only error improvement I would care to try to emit here would be the one that says what one can read in the documentation:

"ERROR:  Default expressions shall not execute queries."

Because of that prohibition this confusing error should be rare enough that if someone breaks the rule and cannot figure out that they did so a bug-report and (hopefully) quick response should suffice; and fare much better in a cost/benefit analysis.

So, no, the error message is not a bug - we don't make any promises about what is going to happen when the rules are broken.  In the rare case that breaking the rule may be a good idea the current state at least allows you to do so while detecting and prohibiting the case at runtime would not...

David J.

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