event trigger should provide more details

On 30/05/2023 22:23 CEST Lian Jiang <jiangok2006@gmail.com> wrote:

I plan to create an event trigger to detect schema change (e.g. add/remove
a column, change column type), and write it into a separate table (e.g.
EVENTS). Then a process periodically reads this table to send schema change
notification. However, the event trigger (https://www.postgresql.org/docs/current/plpgsql-trigger.html)
(43.10.2. Triggers on Events) does not provide me info such as which table
is altered, old and new schema. Am I missing something? Thanks very much for
any hints.

You must use ddl_command_end event triggers[0]https://www.postgresql.org/docs/current/event-trigger-definition.html and call function
pg_event_trigger_ddl_commands[1]https://www.postgresql.org/docs/current/functions-event-triggers.html to get info such as altered table and column.

[0]: https://www.postgresql.org/docs/current/event-trigger-definition.html
[1]: https://www.postgresql.org/docs/current/functions-event-triggers.html

--
Erik

Thanks. This is helpful. Below is the result when I add a column to
public.accounts.

obj.classid, -- 1259
obj.objid, -- 16409
obj.objsubid, -- 0
obj.command_tag, -- ALTER TABLE
obj.object_type, -- table
obj.schema_name, -- public
obj.object_identity, -- public.accounts
obj.in_extension; -- f

The info useful for me is command_tag, object_type, object_identity.
classid, objid is not useful since object_identity is more explicit.
objsubid is not useful because I don't need comment
</messages/by-id/Pine.LNX.4.33.0212091822050.15095-100000@leary.csoft.net&gt;
information for schema change.

Besides table name, I still need:
* which columns are added and their types.
* which columns have type change, the old and new types.
* which columns are dropped.

Will command field provide this info? I don't have an example and decoding
it needs C code
</messages/by-id/20190712222343.GA26924@alvherre.pgsql&gt;
. If I cannot get such info from pg_event_trigger_ddl_commands, I may need
to maintain schema snapshots myself and diff the old and new snapshots upon
an alter table/view event. Which way should I go? Thanks a lot.

On Tue, May 30, 2023 at 2:42 PM Erik Wienhold <ewie@ewie.name> wrote:

On 30/05/2023 22:23 CEST Lian Jiang <jiangok2006@gmail.com> wrote:

I plan to create an event trigger to detect schema change (e.g.

add/remove

a column, change column type), and write it into a separate table (e.g.
EVENTS). Then a process periodically reads this table to send schema

change

notification. However, the event trigger (

https://www.postgresql.org/docs/current/plpgsql-trigger.html)

(43.10.2. Triggers on Events) does not provide me info such as which

table

is altered, old and new schema. Am I missing something? Thanks very much

for

any hints.

You must use ddl_command_end event triggers[0] and call function
pg_event_trigger_ddl_commands[1] to get info such as altered table and
column.

[0] https://www.postgresql.org/docs/current/event-trigger-definition.html
[1] https://www.postgresql.org/docs/current/functions-event-triggers.html

--
Erik

--

Create your own email signature
<https://www.wisestamp.com/signature-in-email/?utm_source=promotion&amp;utm_medium=signature&amp;utm_campaign=create_your_own&amp;srcid=5234462839406592&gt;

On 31/05/2023 00:28 CEST Lian Jiang <jiangok2006@gmail.com> wrote:

The info useful for me is command_tag, object_type, object_identity.
classid, objid is not useful since object_identity is more explicit.
objsubid is not useful because I don't need comment
(/messages/by-id/Pine.LNX.4.33.0212091822050.15095-100000@leary.csoft.net)
information for schema change.

You need objsubid to identify the column in pg_attribute to get its type, not
just to get the comment from pg_description as the linked thread says.

Besides table name, I still need:
* which columns are added and their types.
* which columns have type change, the old and new types.
* which columns are dropped.

Will command field provide this info? I don't have an example and decoding it
needs C code (/messages/by-id/20190712222343.GA26924@alvherre.pgsql).
If I cannot get such info from pg_event_trigger_ddl_commands, I may need to
maintain schema snapshots myself and diff the old and new snapshots upon an
alter table/view event. Which way should I go? Thanks a lot.

Right off the bat, I would combine it with a ddl_command_start event trigger to
record the necessary info (current columns and their types) in a temp table.
Query this table in the ddl_command_end event trigger to figure out which
columns have changes. This can be done entirely in plpgsql without using the
command column.

--
Erik