Subject: Re: Why does TRUNCATE require a special privilege?

From: Dominique Devienne <[email-masked]>
Date: Fri, 16 Jan 2026 11:32:41 +0100

On Fri, Jan 16, 2026 at 10:13 AM Marcelo Fernandes <[email-masked]> wrote:
> From the documentation:
> > TRUNCATE quickly removes all rows from a set of tables. It has the same
> > effect as an unqualified DELETE on each table, but since it does not actually
> > scan the tables it is faster.
> > (...)
> > You must have the TRUNCATE privilege on a table to truncate it.
>
> Granted that TRUNCATE and DELETE are different operations under the hood, but
> why would the TRUNCATE operation require its own specific privilege rather than
> say, use the same privilege as the DELETE operation?

It's kinda obvious, when you read the notes.

1) Not MVCC-safe.
2) Do not fire TRIGGERs, thus breaking data-integrity
3) "Viral" in the presence of FKs, i.e. related tables must also be TRUNCATEd

Just these 3 are HUGE departures from a DELETE. --DD

---

From: Adrian Klaver <[email-masked]>
Date: Fri, 16 Jan 2026 08:46:56 -0800

> 2) Do not fire TRIGGERs, thus breaking data-integrity

It will not fire ON DELETE triggers, it will fire ON TRUNCATE triggers.

> 3) "Viral" in the presence of FKs, i.e. related tables must also be TRUNCATEd

Only if you add the CASCADE option, or TRUNCATE them in the same 
command. Otherwise it will fail.

> Just these 3 are HUGE departures from a DELETE. --DD

I would add from:

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

"TRUNCATE acquires an ACCESS EXCLUSIVE lock on each table it operates 
on, which blocks all other concurrent operations on the table. When 
RESTART IDENTITY is specified, any sequences that are to be restarted 
are likewise locked exclusively. If concurrent access to a table is 
required, then the DELETE command should be used instead."

and

""
When RESTART IDENTITY is specified, the implied ALTER SEQUENCE RESTART 
operations are also done transactionally; that is, they will be rolled 
back if the surrounding transaction does not commit. Be aware that if 
any additional sequence operations are done on the restarted sequences 
before the transaction rolls back, the effects of these operations on 
the sequences will be rolled back, but not their effects on currval(); 
that is, after the transaction currval() will continue to reflect the 
last sequence value obtained inside the failed transaction, even though 
the sequence itself may no longer be consistent with that. This is 
similar to the usual behavior of currval() after a failed transaction."

-- 
Adrian Klaver
[email-masked]

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From: Marcelo Fernandes <[email-masked]>
Date: Sat, 17 Jan 2026 10:15:18 +1300

Those operations do different things, sure.

But from a roles/privilege framework perspective, why would you want to give
certain users the DELETE privilege whereas others you want to give them
only the TRUNCATE privilege?

Are we saying to a user that "You need a different level of privilege because
you are about to cause a MVCC-unsafe operation?".

Or is the privilege framework simply ruling "Do different things, have
different permissions"?

Marcelo.

---

From: Adrian Klaver <[email-masked]>
Date: Fri, 16 Jan 2026 14:11:55 -0800

> you are about to cause a MVCC-unsafe operation?".

The thing I see as difference worthy of separation is:

"CASCADE

     Automatically truncate all tables that have foreign-key references 
to any of the named tables, or to any tables added to the group due to 
CASCADE.
"

Now the same end result can be done with:

delete from some_table:

where the FK's pointing at some_table have ON CASCADE DELETE. The 
difference being a DBA has the option of creating the FKs with ON 
CASCADE NO ACTION which would throw an error. In other words you can 
prevent an unconstrained DELETE on some_table from removing all the 
child records. With TRUNCATE ... CASCADE, you cannot, the potential for 
harm is greater.

---

From: Tom Lane <[email-masked]>
Date: Fri, 16 Jan 2026 18:14:40 -0500

> you are about to cause a MVCC-unsafe operation?".

Personally I think that's a plenty good enough reason ;-).
The very different locking level is another good reason.
TRUNCATE will block all other activity on the table, while
DELETE doesn't.

However, looking at our git history, it appears that TRUNCATE was
originally only permitted to the table owner.  The separate permission
bit was added (years later) so that the owner could grant out the
ability to others, without doing anything as non-backwards-compatible
as reinterpreting what operations a DELETE grant allows.

			regards, tom lane