Re: ALTER TABLE ... NOREWRITE option

On 2012-12-06 18:21:09 +0000, Simon Riggs wrote:
> On 6 December 2012 00:46, Robert Haas <robertmhaas@gmail.com> wrote:
> > On Wed, Dec 5, 2012 at 6:45 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> >> Yes, but it is also the trigger writers problem.
> >
> > Maybe to some degree.  I don't think that a server crash or something
> > like a block-read error is ever tolerable though, no matter how silly
> > the user is with their event trigger logic.  If we go down that road
> > it will be impossible to know whether errors that are currently
> > reliable indicators of software or hardware problems are in fact
> > caused by event triggers.   Of course, if an event trigger causes the
> > system to error out in some softer way, that's perfectly fine...
>
> How are event triggers more dangerous than normal triggers/functions?

Normal triggers aren't run when the catalog is in an in-between state
because they aren't run while catalog modifications are taking place.

Consider a trigger running before CREATE INDEX CONCURRENTLY (which
relies on being the first thing to do database access in a transaction)
that does database access.

Or a trigger running during a table rewrite that inserts into the
intermediary table (pg_rewrite_xxx or whatever they are named). That
possibly would lead to a crash because the pg_class entry of that table
are suddently gone.

Greetings,

Andres Freund

--Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services