Date: Wed, 18 Mar 2020 18:02:13 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Ashutosh Bapat <ashutosh.bapat@2ndquadrant.com>
Cc: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>,
	Pg Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: BEFORE ROW triggers for partitioned tables
Message-ID: <20200318210213.GA9781@alvherre.pgsql>
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On 2020-Mar-17, Ashutosh Bapat wrote:

> On Fri, 13 Mar 2020 at 21:55, Alvaro Herrera <alvherre@2ndquadrant.com>
> wrote:

> > Note that in this implementation I no longer know which column is the
> > problematic one, but I suppose users have clue enough.  Wording
> > suggestions welcome.
> 
> When we have expression as a partition key, there may not be one particular
> column which causes the row to move out of partition. So, this should be
> fine.

True.

> A slight wording suggestion below.
> 
> - * Complain if we find an unexpected trigger type.
> - */
> - if (!TRIGGER_FOR_AFTER(trigForm->tgtype))
> - elog(ERROR, "unexpected trigger \"%s\" found",
> - NameStr(trigForm->tgname));
> 
> !AFTER means INSTEAD OF and BEFORE. Do you intend to allow INSTEAD OF
> triggers as well?

Hmm, yeah, this should check both types; I'll put it back.  Note that
this is just a cross-check that the catalogs we're going to copy don't
contain bogus info; the real backstop for that at the user level is in
the other one you complain about:

> - */
> - if (stmt->timing != TRIGGER_TYPE_AFTER)
> 
> Same comment as the above?

Note that in this one we have a check for INSTEAD before we enter the
FOR EACH ROW block, so this case is already covered -- AFAICS the code
is correct.

> + /*
> + * After a tuple in a partition goes through a trigger, the user
> + * could have changed the partition key enough that the tuple
> + * no longer fits the partition.  Verify that.
> + */
> + if (trigger->tgisclone &&
> 
> Why do we want to restrict this check only for triggers which are
> cloned from the ancestors?

Because it's not our business in the other case.  When the trigger is
defined directly in the partition, it's the user's problem if something
going amiss.

> + !ExecPartitionCheck(relinfo, slot, estate, false))
> + ereport(ERROR,
> + (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
> + errmsg("moving row to another partition during a BEFORE trigger is not
> supported"),
> + errdetail("Before trigger \"%s\", row was to be in partition \"%s.%s\"",
> 
> In the error message you removed above, we are mentioning BEFORE FOR EACH
> ROW trigger. Should we continue to use the same terminology?

Sounds good, I'll change that.

I also changed the errdetail slightly:
	errdetail("Before executing trigger \"%s\", the row was to be in partition \"%s.%s\"",

> I was wondering whether it would be good to check the partition
> constraint only once i.e. after all before row triggers have been
> executed. This would avoid throwing an error in case multiple triggers
> together worked to keep the tuple in the same partition when
> individual trigger/s caused it to move out of that partition. But then
> we would loose the opportunity to point out the before row trigger
> which actually caused the row to move out of the partition. Anyway,
> wanted to bring that for the discussion here.

Yeah, I too thought about a combination of triggers that move the tuple
elsewhere and back.  Frankly, I don't think we need to support that.  It
sounds devious and likely we'll miss some odd corner case -- anything
involving the weird cross-partition UPDATE mechanism sounds easy to get
wrong.

> +-- Before triggers and partitions
> 
> The test looks good. Should we add a test for partitioned table with
> partition
> key as expression?

Will do.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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