Subject: Trigger behaviour not as stated

From: [email-masked]
Date: Wed, 29 Nov 2017 19:39:34 +0000

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/10/static/sql-createtrigger.html
Description:

URL: https://www.postgresql.org/docs/current/static/sql-createtrigger.html

Statement: &quot;In contrast, row-level triggers are fired for all affected
partitions or child tables.&quot;

Row-level triggers are not fired on child tables where the trigger ON BEFORE
UPDATE | DELETE is on the parent table. Only works on BEFORE INSERT.

---

From: Bruce Momjian <[email-masked]>
Date: Wed, 24 Jan 2018 13:10:08 -0500

> UPDATE | DELETE is on the parent table. Only works on BEFORE INSERT.

Uh, can you email us an example of the failure so we can research it? 
Thanks.

-- 
  Bruce Momjian  <[email-masked]>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +

---

From: Bruce Momjian <[email-masked]>
Date: Wed, 24 Jan 2018 17:07:01 -0500

> Uh, can you email us an example of the failure so we can research it? 

OK, I have some more details on this.  First there is the Stackoverflow
report:

  https://stackoverflow.com/questions/47557665/postgresql-on-before-delete-trigger-not-firing-on-a-parent-table-in-an-inheritan

The report confirms that row-level triggers are fired _only_ on affected
tables (meaning the table that had a row change), not on any table
mentioned _or_ affected.  The current wording, added in this commit:

	commit 501ed02cf6f4f60c3357775eb07578aebc912d3a
	Author: Andrew Gierth <[email-masked]>
	Date:   Wed Jun 28 18:55:03 2017 +0100
	
	    Fix transition tables for partition/inheritance.
	
	    We disallow row-level triggers with transition tables on child tables.
	    Transition tables for triggers on the parent table contain only those
	    columns present in the parent.  (We can't mix tuple formats in a
	    single transition table.)
	
	    Patch by Thomas Munro
	
	    Discussion: https://postgr.es/m/CA%2BTgmoZzTBBAsEUh4MazAN7ga%3D8SsMC-Knp-6cetts9yNZUCcg%40mail.gmail.com

should be improved.  The attached patch updates the docs to say
statement-level triggers fire on the "referenced" table, while row-level
triggers fire only on the "affected" table, (vs. all affected tables)
even if they are not referenced in the query.  I would backpatch this to
PG 10.

The second attachment is an SQL query script that illustrates the
behavior.

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +
diff --git a/doc/src/sgml/ref/create_trigger.sgml b/doc/src/sgml/ref/create_trigger.sgml
new file mode 100644
index a8c0b57..9cd369d
*** a/doc/src/sgml/ref/create_trigger.sgml
--- b/doc/src/sgml/ref/create_trigger.sgml
*************** UPDATE OF <replaceable>column_name1</rep
*** 501,509 ****
  
    <para>
     Modifying a partitioned table or a table with inheritance children fires
!    statement-level triggers directly attached to that table, but not
     statement-level triggers for its partitions or child tables.  In contrast,
!    row-level triggers are fired for all affected partitions or child tables.
     If a statement-level trigger has been defined with transition relations
     named by a <literal>REFERENCING</literal> clause, then before and after
     images of rows are visible from all affected partitions or child tables.
--- 501,510 ----
  
    <para>
     Modifying a partitioned table or a table with inheritance children fires
!    statement-level triggers directly attached to the referenced table, but not
     statement-level triggers for its partitions or child tables.  In contrast,
!    row-level triggers are fired only on affected partitions or child tables,
!    even if they are not referenced in the query.
     If a statement-level trigger has been defined with transition relations
     named by a <literal>REFERENCING</literal> clause, then before and after
     images of rows are visible from all affected partitions or child tables.

---

From: Thomas Munro <[email-masked]>
Date: Sun, 28 Jan 2018 23:19:47 +1300

> even if they are not referenced in the query.  I would backpatch this to

+1

I was trying to convey that, but it does seem a little terse and
cryptic.  Your addition of "referenced" and "only" make it clearer.

-- 
Thomas Munro
http://www.enterprisedb.com

---

From: Tom Lane <[email-masked]>
Date: Sun, 28 Jan 2018 14:18:38 -0500

Thomas Munro <[email-masked]> writes:
>> ... The attached patch updates the docs to say

> cryptic.  Your addition of "referenced" and "only" make it clearer.

Hm, the first part of Bruce's change seems fine, but I think this wording:

     ...  In contrast,

is still confusing.  How about something like

     In contrast, row-level triggers are fired for each actual row change,
     including changes in partitions or child tables that are not directly
     named in the query.

Possibly "row operation" would be better than "row change".

			regards, tom lane

---

From: Bruce Momjian <[email-masked]>
Date: Sun, 28 Jan 2018 15:34:02 -0500

> Possibly "row operation" would be better than "row change".

Uh, I don't think we want to highlight the statement vs row difference
here but the fact that statement triggers fire on the referenced object
and not on the effected rows.  I have attached an updated patch which I
think is an improvement.

index a8c0b57..42649d2
  
    <para>
  
    <para>
     Modifying a partitioned table or a table with inheritance children fires
!    statement-level triggers directly attached to the referenced table, but not
     statement-level triggers for its partitions or child tables.  In contrast,
!    row-level triggers are fired on the rows in effected partitions or
!    child tables, even if they are not referenced in the query.
     If a statement-level trigger has been defined with transition relations
     named by a <literal>REFERENCING</literal> clause, then before and after
     images of rows are visible from all affected partitions or child tables.

---

From: Tom Lane <[email-masked]>
Date: Sun, 28 Jan 2018 15:57:07 -0500

Bruce Momjian <[email-masked]> writes:
> think is an improvement

I still think that that's not well written.  A large part of the confusion
here is over what "referenced" means.  To my mind, child tables/partitions
are referenced by an inherited query, just not explicitly.  So that's why
I'd prefer wording like "directly named in the query" (or "explicitly
named").  If you insist on using "referenced" you could write "explicitly
referenced", but IMO that's longer and no clearer.

A lesser complaint is that this reads like the antecedent of "they" is the
rows, not the tables containing them, making the meaning of "referenced"
even less clear.

Maybe something like

    In contrast, row-level triggers are fired for individual row change
    events, and the triggers that are fired for an event are those
    attached to the specific table containing the changed row, even if
    it is a partition or child table not directly named in the query.

			regards, tom lane

---

From: Bruce Momjian <[email-masked]>
Date: Sun, 28 Jan 2018 17:58:24 -0500

>     it is a partition or child table not directly named in the query.

Oh, I am sorry.  I was focused on the first part of the sentence and
didn't notice your change to the second part.  How is this attachment?

index a8c0b57..6519757
  
    <para>
  
    <para>
!    child tables, even if they are not explicitly named in the query.

---

From: Tom Lane <[email-masked]>
Date: Sun, 28 Jan 2018 18:12:01 -0500

> didn't notice your change to the second part.  How is this attachment?

Seems same as your previous version?

			regards, tom lane

---

From: Bruce Momjian <[email-masked]>
Date: Sun, 28 Jan 2018 18:17:19 -0500

> Seems same as your previous version?

OK, new vesion that uses "explicitly named" in both modified doc lines.

index a8c0b57..dab1041
  
    <para>
  
    <para>
!    statement-level triggers attached to the explicitly named table, but not

---

From: "Ian R. Campbell" <[email-masked]>
Date: Mon, 29 Jan 2018 11:32:34 +0000

The original confusion on this is answered in part by the following
statement (taken from the answer to my SO question):

"UPDATEs and DELETEs on the parent table will affect rows in the child
tables (if you don't specify ONLY), but triggers will only be fired by data
modifications directed directly against the table with the trigger on it "

This clearly explains that a trigger attached to a parent table will not
fire if the data being modified is in a child table.

The second part of the confusion is that INSERT is not considered to be a
row modification and will fire a BEFORE INSERT trigger on the parent table
even when the data goes into a child (whereas UPDATE and DELETE will not
fire a parent trigger).

My proposal (adapted from Tom's):

 In contrast, row-level UPDATE and DELETE triggers are fired for individual
row change
    events only on the table to which the trigger is attached. Therefore,
UPDATE and DELETE triggers on
   a parent table will only fire when rows in the parent table are being
modified. Likewise,
   UPDATE and DELETE triggers on a child table will only fire when rows in
the child table are being modified.
   Note that INSERT statements do not follow these update rules, so
statements run on parent tables will
   insert rows in child tables if the trigger function so directs.

The last line may need a bit of work, but I feel the text above it is clear.

Ian

---

From: Bruce Momjian <[email-masked]>
Date: Mon, 29 Jan 2018 10:28:32 -0500

On Mon, Jan 29, 2018 at 11:32:34AM +0000, Ian R. Campbell wrote:
> The second part of the confusion is that INSERT is not considered to be a row
> modification and will fire a BEFORE INSERT trigger on the parent table even
> when the data goes into a child (whereas UPDATE and DELETE will not fire a
> parent trigger).

Ian, that is not true based on my testing.  Running that attached script
that I already posted shows:

	test=> INSERT INTO parent VALUES (1, 'one');
	NOTICE:  Called by parent INSERT
	INSERT 0 1
	test=> INSERT INTO child VALUES (2, 'two');
-->	NOTICE:  Called by child INSERT
	INSERT 0 1

---

From: Bruce Momjian <[email-masked]>
Date: Wed, 31 Jan 2018 17:00:54 -0500

> OK, new vesion that uses "explicitly named" in both modified doc lines.

Applied and backpatched through 10, where the text was added.