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Re: Command Triggers, patch v11

From: Thom Brown <thom(at)linux(dot)com>
To: Dimitri Fontaine <dimitri(at)2ndquadrant(dot)fr>
Cc: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Command Triggers, patch v11
Date: 2012-02-24 22:32:35
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On 24 February 2012 22:04, Dimitri Fontaine <dimitri(at)2ndquadrant(dot)fr> wrote:
> Hi,
> Please find attached the latest version of the command triggers patch,
> in context diff format, with support for 79 commands and documentation
> about why only those, and with some limitations explained.
> I also cleaned up the node function support business that was still in
> there from the command rewriting stuff that we dropped, and did a merge
> from tonight's master branch (one of a few clean merges).
> This is now the whole of it, and I continue being available to make any
> necessary change, although I expect only minor changes now.  Thanks to
> all reviewers and participants into the previous threads, you all have
> allowed me to reach the current point by your precious advice, comments
> and interest.
> The patch implements :
>  - BEFORE/AFTER ANY command triggers
>  - BEFORE/AFTER command triggers for 79 documented commands
>  - regression tests exercised by the serial schedule only
>  - documentation updates with examples
> That means you need to `make installcheck` here. Installing the tests in
> the parallel schedule does not lead to consistent output as installing a
> command trigger will impact any other test using that command, and the
> output becomes subject to the exact ordering of the concurrent tests.
> The only way for a BEFORE command triggers to change the command's
> behaviour is by raising an exception that aborts the whole transaction.
> Command triggers are called with the following arguments:
>  - the “event” (similar to TG_WHEN, either 'BEFORE' or 'AFTER')
>  - the command tag (the real one even when an ANY trigger is called)
>  - the object Id if available (e.g. NULL for a CREATE statement)
>  - the schema name (can be NULL)
>  - the object name (can be NULL)
> When the trigger's procedure we're calling is written in C, then another
> argument is passed next, which is the parse tree Node * pointer.
> I've been talking with Marko Kreen about supporting ALTER TABLE and such
> commands automatically in Londiste: given that patch, it requires
> writing code in C that will rewrite the command string.  It so happens
> that I already have worked on that code, so we intend on bringing
> support for ALTER TABLE and other commands in Skytools 3 for 9.2.
> I think the support code should be made into an extension that both
> Skytools and Slony would be able to share. The extension code will be
> able to adapt to major versions changes as they are released.  Bucardo
> would certainly be interested too, we could NOTIFY it from such an
> extension.  The design is yet to be done here, but it's clearly possible
> to implement such a feature given the current patch.
> The ANY trigger support is mainly there to allow people to stop any DDL
> traffic on their databases, then allowing it explicitly with an ALTER
> COMMAND TRIGGER ... SET DISABLE when appropriate only.  To that
> end, the ANY command trigger is supporting more commands than you can
> attach specific triggers too.
> It's also possible to ENABLE a command trigger on the REPLICA only
> thanks to the session_replication_role GUC.  Support for command
> triggers on an Hot Standby node is not provided in this patch.

Hi Dimitri,

I just tried building the docs with your patch and it appears
doc/src/sgml/ref/allfiles.sgml hasn't been updated with the necessary
references for alterCommandTrigger, createCommandTrigger and

Also in ref/alter_command_trigger.sgml, you define SQL-CREATETRIGGER.
Shouldn't this be SQL-CREATECOMMANDTRIGGER?  And there also appears to
be orphaned text in the file too, such as "Forbids the execution of
any DDL command".  And there's a stray </para> on line 299.

I attach updated versions of both of those files, which seems to fix
all these problems.


Attachment: allfiles.sgml
Description: text/sgml (9.8 KB)
Attachment: create_command_trigger.sgml
Description: text/sgml (9.5 KB)

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