Thread: Event Triggers unable to capture the DDL script executed

On Wed, 2023-02-22 at 07:57 +0000, Neethu P wrote:
> We are using event triggers to capture the DDL changes in a postgres database.
> However, we are unable to get the column information & the actual DDL script
> executed, while a table is altered. 
> Also, in the postgres documentation for pg_event_trigger_ddl_commands ()- it is mentioned as below.
> pg_ddl_command    A complete representation of the command, in internal
> format. Thiscannot be output directly, but it can be passed to other functions
> to obtain different pieces of information about the command.
>
> Is it possible to access pg_ddl_command in postgresql? Or is there any scripts
> which can help to get theactual Alter DDL statement that was executed by the user? 

That is simple if you write the event trigger in C.  I would say that that is the
only way to get at the actual statement.

Yours,
Laurenz Albe

>>-----Исходное сообщение-----
>>От: Laurenz Albe <laurenz.albe@cybertec.at>
>>Отправлено: 22 февраля 2023 г. 12:52
>>Кому: Neethu P <neeth_3@hotmail.com>; pgsql-general <pgsql-general@postgresql.org>
>>Тема: Re: Event Triggers unable to capture the DDL script executed

>>On Wed, 2023-02-22 at 07:57 +0000, Neethu P wrote:
>>> We are using event triggers to capture the DDL changes in a postgres database.
>>> However, we are unable to get the column information & the actual DDL
>>> script executed, while a table is altered.
>>> Also, in the postgres documentation for pg_event_trigger_ddl_commands ()- it is mentioned as below.
>>> pg_ddl_command    A complete representation of the command, in internal
>>> format. Thiscannot be output directly, but it can be passed to other
>>> functions to obtain different pieces of information about the command.
>>>
>>> Is it possible to access pg_ddl_command in postgresql? Or is there any
>>> scripts which can help to get theactual Alter DDL statement that was executed by the user?

>>That is simple if you write the event trigger in C.  I would say that that is the only way to get at the actual
statement.

>>Yours,
>>Laurenz Albe



In MSSQL there is a brilliant possibility to have a server wide trigger to monitor commands. We are using It to have a
historyfor all DDL operations. 

Please try this (on new empty database) and give a feedback.

CREATE OR REPLACE FUNCTION public.notice_ddl()
    RETURNS event_trigger
    LANGUAGE 'plpgsql'
    COST 100
    VOLATILE NOT LEAKPROOF
AS $BODY$
DECLARE
    r RECORD;
begin
    raise info '%', session_user || ' ran '||tg_tag||' '||current_query();
    FOR r IN SELECT * FROM pg_event_trigger_ddl_commands() LOOP
        RAISE NOTICE 'we got a % event for object " %"',
            r.command_tag, r.object_identity;
    END LOOP;
end;
$BODY$;

CREATE OR REPLACE FUNCTION public.notice_ddl_drop()
    RETURNS event_trigger
    LANGUAGE 'plpgsql'
    COST 100
    VOLATILE NOT LEAKPROOF
AS $BODY$
DECLARE
    r RECORD;
begin
    raise info '%', session_user || ' ran '||tg_tag||' '||current_query();
    FOR r IN SELECT * FROM pg_event_trigger_dropped_objects()
    LOOP
        RAISE NOTICE 'dropped: type "%" identity %',
                r.object_type, r.object_identity;
    END LOOP;
end;
$BODY$;


CREATE EVENT TRIGGER etg ON DDL_COMMAND_END
    EXECUTE PROCEDURE public.notice_ddl();

CREATE EVENT TRIGGER etg_drop ON SQL_DROP
    EXECUTE PROCEDURE public.notice_ddl_drop();

On Thu, 2023-02-23 at 04:10 +0000, Neethu P wrote:
> Actually, current_query() may not help us in our case, as we won't be able
> to capture the ddl statement completely in case if it's in multiple lines.

Multiple lines should not be a problem.  The problems I see are

- you won't catch DDL statements issued in a function with that, since you
  only see the top-level statement

- if you have the DDL statement as string, you need to parse it, which is
  non-trivial

> Can you please help me with the event trigger in C? & also how can we
> integrate it with our current postgresql DB?

In an e-mail, I cannot do much beyond pointing you to the documentation:
https://www.postgresql.org/docs/current/event-trigger-interface.html
There is also a simple example:
https://www.postgresql.org/docs/current/event-trigger-example.html

Getting used to reading and writing PostgreSQL server code takes a while.

Yours,
Laurenz Albe

On Mon, 2023-02-27 at 03:52 +0000, Neethu P wrote:
> Is it possible to access the pg_ddl_command using C function? In the shared links,
> I couldn't find one. Can you please share an example for the same?

No.  You could hire a professional.

Yours,
Laurenz Albe