Обсуждение: 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] 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

> 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
> (https://www.postgresql.org/message-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 (https://www.postgresql.org/message-id/20190712222343.GA26924%40alvherre.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