RE: [EXTERNAL]: Re: [EXTERNAL]: Re: UPSERT in Postgres

Hi Alban, 

"I am not expecting an error here", by "here" I means when doing a TRUE UPSERT (an upsert current does not exist in
Postgres).
 
I am NOT referring to an "Insert on conflict do update" (which despite its intention and wide acceptance is not fully
equivalentto a true upsert).
 
I understand the error I am getting now is due to not null constraint given how "insert on conflict" works. 

An UPSERT checks whether a row exists, if so, it does an update, if not it does an insert. This is the literal
definition.
An UPSERT is NOT defined as try do an INSERT first, if violate uniqueness constraint, do update. This is what on
conflictdo update is doing. 
 
We cannot define UPSERT with what "ON CONFLICT DO UPDATE" is doing. That is a logical fallacy.
 UPSERT is a higher-level abstract concept. ON CONFLICT DO UPDATE is an implementation of UPSERT. not the other way
around.

When doing a true UPSERT, if a row already exists, there is no need provide an (not null) column, since I am doing an
update.
 

With `UPSERT person (id, is_active)` VALUES (0, true). Is it necessary to provide the not null "name" column here
logically?
 
Not really, I already specified the row with the `id` column, then I specify the column I want to update `is_active`. 
* the id does exist; the update can be executed without any issue or assumptions
* the id does not exit; then I am expecting a violate not null constraint. 

On contrast, with `INSERT person (id, is_active) value (0, true) ON CONFLICT DO UPDATE set is_active=true`,
this statement will always fail regardless of whether the id exists or not. 
So the behavior of ON CONFLICT DO UPDATE differs from a UPSERT with the presence of a not null column. 
Hence why ON CONFLICT DO UPDATE is not complete equivalent to a TRUE upsert.

Cheers,
Louis Tian


-----Original Message-----
From: Alban Hertroys <haramrae@gmail.com> 
Sent: Sunday, April 9, 2023 7:26 PM
To: Louis Tian <louis.tian@aquamonix.com.au>
Cc: Peter Geoghegan <pg@bowt.ie>; pgsql-general@lists.postgresql.org
Subject: [EXTERNAL]: Re: [EXTERNAL]: Re: UPSERT in Postgres

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> On 7 Apr 2023, at 2:49, Louis Tian <louis.tian@aquamonix.com.au> wrote:

(…)

> I am not expecting an error here. The problem is with no conflict it always go down the insert path first and results
ina not null constraint error.
 
> While I am expecting the insert is never executed in the first place when that row already exist (as identified by it
primarykey). So the update execute without error.
 
> I hope the pesudo code above is enough to clarify the difference?

Your assumption on what the problem is, is not correct. The problem is not with the conflict resolution, it is with
yourstatement violating a not null constraint.
 

It doesn’t matter whether you insert first or update first, either operation is going to violate that constraint.
You’respecifying a NULL value for a column that doesn’t accept that because it has a NOT NULL constraint. That is your
problem.

Alban Hertroys
--
There is always an exception to always.




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