Date: Wed, 12 Jan 2022 16:39:20 -0300
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Andrei Matei <andreimatei1@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@lists.postgresql.org
Subject: Re: is ErrorResponse possible on Sync?
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On 2022-Jan-12, Andrei Matei wrote:

> If Sync itself cannot fail, then what is this
> sentence really saying: "This parameterless message (ed. Sync) causes the
> backend to close the current transaction if it's not inside a BEGIN/COMMIT
> transaction block (“close” meaning to commit if no error, or roll back if
> error)." ?
> This seems to say that, outside of BEGIN/END, the transaction is committed
> at Sync time (i.e. if the Sync is never sent, nothing is committed).
> Presumably, committing a transaction can fail even if no
> previous command/statement failed, right?

A deferred trigger can cause a failure at COMMIT time for which no
previous error was reported.

alvherre=# create table t (a int unique deferrable initially deferred);
CREATE TABLE
alvherre=# insert into t values (1);
INSERT 0 1
alvherre=# begin;
BEGIN
alvherre=*# insert into t values (1);
INSERT 0 1
alvherre=*# commit;
ERROR:  duplicate key value violates unique constraint "t_a_key"
DETALLE:  Key (a)=(1) already exists.

I'm not sure if you can cause this to explode with just a Sync message, though.

-- 
Álvaro Herrera              Valdivia, Chile  —  https://www.EnterpriseDB.com/