Re: cataloguing NOT NULL constraints
От | Tom Lane |
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Тема | Re: cataloguing NOT NULL constraints |
Дата | |
Msg-id | 3801207.1681057430@sss.pgh.pa.us обсуждение исходный текст |
Ответ на | Re: cataloguing NOT NULL constraints (Andres Freund <andres@anarazel.de>) |
Ответы |
Re: cataloguing NOT NULL constraints
(Alvaro Herrera <alvherre@alvh.no-ip.org>)
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Список | pgsql-hackers |
Andres Freund <andres@anarazel.de> writes: > I think > https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2023-04-07%2021%3A16%3A04 > might point out a problem with the pg_dump or pg_upgrade backward compat > paths: Yeah, this patch has broken every single upgrade-from-back-branch test. I think there's a second problem, though: even without considering back branches, this has changed pg_dump output in a way that I fear is unacceptable. Consider for instance this table definition (from rules.sql): create table rule_and_refint_t1 ( id1a integer, id1b integer, primary key (id1a, id1b) ); This used to be dumped as CREATE TABLE public.rule_and_refint_t1 ( id1a integer NOT NULL, id1b integer NOT NULL ); ... ... load data ... ... ALTER TABLE ONLY public.rule_and_refint_t1 ADD CONSTRAINT rule_and_refint_t1_pkey PRIMARY KEY (id1a, id1b); In the new dispensation, pg_dump omits the NOT NULL clauses. Great, you say, that makes the output more like what the user wrote. I'm not so sure. This means that the ALTER TABLE will be compelled to perform a full-table scan to verify that there are no nulls in the already-loaded data before it can add the missing NOT NULL constraint. The old dump output was carefully designed to avoid the need for that scan. Admittedly, we have to do a scan anyway to build the index, so this is strictly less than a 2X penalty on the ALTER, but is that acceptable? It might be all right in the context of regular dump/restore, where we're surely doing a lot of per-row work anyway to load the data and make the index. In the context of pg_upgrade, though, it seems absolutely disastrous: there will now be a per-row cost where there was none before, and that is surely a deal-breaker. BTW, I note from testing that the NOT NULL clauses *are* still emitted in at least some cases when doing --binary-upgrade from an old version. (This may be directly related to the buildfarm failures, not sure.) That's no solution though, because now what you get in pg_constraint will differ depending on which way you upgraded, which seems unacceptable too. I'm inclined to think that this idea of suppressing the implied NOT NULL from PRIMARY KEY is a nonstarter and we should just go ahead and make such a constraint. Another idea could be for pg_dump to emit the NOT NULL, load data, do the ALTER ADD PRIMARY KEY, and then ALTER DROP NOT NULL. In any case, I wonder whether that's the sort of redesign we should be doing post-feature-freeze. It might be best to revert and try again in v17. regards, tom lane
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