Re: automatic restore point

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>
Cc: Michael Paquier <michael(at)paquier(dot)xyz>, "Yotsunaga, Naoki" <yotsunaga(dot)naoki(at)jp(dot)fujitsu(dot)com>, 'Postgres hackers' <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: automatic restore point
Date: 2018-10-01 22:06:32
Message-ID: 20181001220632.7a4jnzph2cwntq6w@alvherre.pgsql
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On 2018-Oct-01, Peter Eisentraut wrote:

> The following appears to work:
> CREATE FUNCTION evt_automatic_restart_point() RETURNS event_trigger
> LANGUAGE plpgsql
> AS $$
> PERFORM pg_create_restore_point(tg_tag);
> $$;
> CREATE EVENT TRIGGER automatic_restart_point ON ddl_command_start
> EXECUTE PROCEDURE evt_automatic_restart_point();
> Are there any concerns about this?


Re-reading the implementation in standard_ProcessUtility, I wonder what
is PROCESS_UTILITY_QUERY_NONATOMIC -- there seems to be a maze through
SPI that determines whether this flag is set or not, which could affect
whether the event trigger is useful. Are utilities executed through a
procedure detected by event triggers? If so, then this mechanism seems
good enough to me. But if there's a way to sneak utility commands (DROP
TABLE) without the event trigger being invoked, then no (and in that
case maybe it's just a bug in procedures and we can still not include
this patch).

(Grepping for "atomic" is unsurprisingly unhelpful. I really wish we
didn't use plain words as struct member names ...)

On the TRUNCATE case it's a bit annoying that you can't do it with event
triggers -- you have to create individual regular triggers on truncate
for each table (so you probably need yet another event trigger that
creates such triggers).

Álvaro Herrera
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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