The question was asked relatively recently on irc. And it proved to be non-trivial.
Surely, if you want to have one row per category (one address per user), it's trivial – add user_id column to addresses, make it unique, and we're done. But what if we want to allow five addresses? Or five thousands?
Continue reading How to limit rows to at most N per category
On 14th of November 2020, Tom Lane committed patch:
Provide the OR REPLACE option for CREATE TRIGGER.
This is mostly straightforward. However, we disallow replacing
constraint triggers or changing the is-constraint property; perhaps
that can be added later, but the complexity versus benefit tradeoff
doesn't look very good.
Also, no special thought is taken here for whether replacing an
existing trigger should result in changes to queued-but-not-fired
trigger actions. We just document that if you're surprised by the
results, too bad, don't do that. (Note that any such pending trigger
activity would have to be within the current session.)
Takamichi Osumi, reviewed at various times by Surafel Temesgen,
Peter Smith, and myself
Continue reading Waiting for PostgreSQL 14 – Provide the OR REPLACE option for CREATE TRIGGER.
On 18th of March 2020, Alvaro Herrera committed patch:
Enable BEFORE row-level triggers for partitioned tables
... with the limitation that the tuple must remain in the same
Reviewed-by: Ashutosh Bapat
Continue reading Waiting for PostgreSQL 13 – Enable BEFORE row-level triggers for partitioned tables
Another one missed, quite a long time ago, too..:
On 4th of November 2016, Kevin Grittner committed patch:
Implement syntax for transition tables in AFTER triggers.
This is infrastructure for the complete SQL standard feature. No
support is included at this point for execution nodes or PLs. The
intent is to add that soon.
As this patch leaves things, standard syntax can create tuplestores
to contain old and/or new versions of rows affected by a statement.
References to these tuplestores are in the TriggerData structure.
C triggers can access the tuplestores directly, so they are usable,
but they cannot yet be referenced within a SQL statement.
Continue reading Waiting for PostgreSQL 10 – Implement syntax for transition tables in AFTER triggers.
I had two month delay related to some work, but now I can finally write about:
On 7th of December, Robert Haas committed patch:
Implement table partitioning.
Table partitioning is like table inheritance and reuses much of the
existing infrastructure, but there are some important differences.
The parent is called a partitioned table and is always empty; it may
not have indexes or non-inherited constraints, since those make no
sense for a relation with no data of its own. The children are called
partitions and contain all of the actual data. Each partition has an
implicit partitioning constraint. Multiple inheritance is not
allowed, and partitioning and inheritance can't be mixed. Partitions
can't have extra columns and may not allow nulls unless the parent
does. Tuples inserted into the parent are automatically routed to the
correct partition, so tuple-routing ON INSERT triggers are not needed.
Tuple routing isn't yet supported for partitions which are foreign
tables, and it doesn't handle updates that cross partition boundaries.
Currently, tables can be range-partitioned or list-partitioned. List
partitioning is limited to a single column, but range partitioning can
involve multiple columns. A partitioning "column" can be an
Because table partitioning is less general than table inheritance, it
is hoped that it will be easier to reason about properties of
partitions, and therefore that this will serve as a better foundation
for a variety of possible optimizations, including query planner
optimizations. The tuple routing based which this patch does based on
the implicit partitioning constraints is an example of this, but it
seems likely that many other useful optimizations are also possible.
Amit Langote, reviewed and tested by Robert Haas, Ashutosh Bapat,
Amit Kapila, Rajkumar Raghuwanshi, Corey Huinker, Jaime Casanova,
Rushabh Lathia, Erik Rijkers, among others. Minor revisions by me.
Continue reading Waiting for PostgreSQL 10 – Implement table partitioning.
On 5th of April, Simon Riggs committed patch:
Reduce lock levels of some trigger DDL and add FKs
Reduce lock levels to ShareRowExclusive for the following SQL
CREATE TRIGGER (but not DROP or ALTER)
ALTER TABLE ENABLE TRIGGER
ALTER TABLE DISABLE TRIGGER
ALTER TABLE … ADD CONSTRAINT FOREIGN KEY
Original work by Simon Riggs, extracted and refreshed by Andreas Karlsson
New test cases added by Andreas Karlsson
Reviewed by Noah Misch, Andres Freund, Michael Paquier and Simon Riggs
Continue reading Waiting for 9.5 – Reduce lock levels of some trigger DDL and add FKs
On 4th of March, Alvaro Herrera committed patch:
auto_explain: Add logging of trigger execution
Author: Kyotaro HORIGUCHI
Reviewed-by: Jaime Casanova
Continue reading Waiting for 9.4 – auto_explain: Add logging of trigger execution
On 11th of December, Peter Eisentraut committed patch:
PL/Perl: Add event trigger support
From: Dimitri Fontaine <dimitri@2ndQuadrant.fr>
Continue reading Waiting for 9.4 – PL/Perl: Add event trigger support
Some people are afraid of triggers.
Reasons for this are not really understandable for me, but I guess it stems from the fact that these are usually application developers, and not database admins. Or they encountered some kind of problem with triggers, and now they tend to think that triggers are inherently evil.
But they are not.
As virtually anything, triggers have some benefits, and some drawbacks. With a bit of thinking you can use them to do really cool things. But first you have to understand what exactly trigger is, how it works, and when to use which kind.
Continue reading How I Learned to Stop Worrying and Love the Triggers
On 20th of July, Robert Haas committed patch:
Make new event trigger facility actually do something.
Commit 3855968f328918b6cd1401dd11d109d471a54d40 added syntax, pg_dump,
psql support, and documentation, but the triggers didn't actually fire.
With this commit, they now do. This is still a pretty basic facility
overall because event triggers do not get a whole lot of information
about what the user is trying to do unless you write them in C; and
there's still no option to fire them anywhere except at the very
beginning of the execution sequence, but it's better than nothing,
and a good building block for future work.
Along the way, add a regression test for ALTER LARGE OBJECT, since
testing of event triggers reveals that we haven't got one.
Dimitri Fontaine and Robert Haas
This was preceded (two days earlier) by commit, also by Robert Haas, which stated:
Syntax support and documentation for event triggers.
They don't actually do anything yet; that will get fixed in a
follow-on commit. But this gets the basic infrastructure in place,
including CREATE/ALTER/DROP EVENT TRIGGER; support for COMMENT,
SECURITY LABEL, and ALTER EXTENSION .. ADD/DROP EVENT TRIGGER;
pg_dump and psql support; and documentation for the anticipated
initial feature set.
Dimitri Fontaine, with review and a bunch of additional hacking by me.
Thom Brown extensively reviewed earlier versions of this patch set,
but there's not a whole lot of that code left in this commit, as it
Continue reading Waiting for 9.3 – Event triggers