Subject: Re: TRUNCATE ONLY with foreign keys and triggers disabled

From: Dimitrios Apostolou <[email-masked]>
Date: Mon, 14 Apr 2025 16:53:20 +0200 (CEST)

On Mon, 14 Apr 2025, Tom Lane wrote:

> Dimitrios Apostolou <[email-masked]> writes:
>> While doing TRUNCATE ONLY I get:
>>    ERROR: cannot truncate a table referenced in a foreign key constraint
>> But in my case the table to be truncated is already empty, and the
>> TRIGGERS are disabled in all tables.
>
> IIRC, it will let you do it if you truncate both the referenced and
> referencing tables in the same command.  The state of the triggers
> is not material to this, since TRUNCATE doesn't fire them anyway.

Thanks Tom, however that is not possible in my case, the referenced table
is possibly just populated and potentially very big (partitioned table
with many sub-partitions).

Context is that I've modified pg_restore to accept --data-only --clean,
and issues a TRUNCATE ONLY before each table's COPY, in the same
transaction (thus avoiding WAL too). Will send an RFC patch to
pgsql-hackers when I verify it works.

Dimitris

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From: Dimitrios Apostolou <[email-masked]>
Date: Mon, 14 Apr 2025 17:05:12 +0200 (CEST)

On Mon, 14 Apr 2025, Dimitrios Apostolou wrote:

> Thanks Tom, however that is not possible in my case, the referenced table is 
> possibly just populated and potentially very big (partitioned table with many 
> sub-partitions).

Terminology correction: I meant the *referencing* table has just been 
populated. I'm trying to delete the *referenced* table and I get the 
error.

Dimitris

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From: Laurenz Albe <[email-masked]>
Date: Mon, 14 Apr 2025 17:07:58 +0200

On Mon, 2025-04-14 at 17:05 +0200, Dimitrios Apostolou wrote:
> I meant the *referencing* table has just been 

That would break the foreign key constraint, right?
PostgreSQL cannot allow that.

Yours,
Laurenz Albe

---

From: Adrian Klaver <[email-masked]>
Date: Mon, 14 Apr 2025 08:13:56 -0700

> PostgreSQL cannot allow that.

I believe the OP is disabling all triggers including system ones if I 
follow correctly and possibly running a foul of;

https://www.postgresql.org/docs/current/sql-altertable.html

" Disabling or enabling internally generated constraint triggers 
requires superuser privileges; it should be done with caution since of 
course the integrity of the constraint cannot be guaranteed if the 
triggers are not executed."

-- 
Adrian Klaver
[email-masked]