Re: rules on INSERT can't UPDATE new instance?

> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > I thought an INSERT rule with an UPDATE action would work on the same
> > table, but that fails.  Seems the rule is firing before the INSERT
> > happens.
>
> Yes, a trigger is the right way to do surgery on a tuple before it is
> stored.  Rules are good for generating additional SQL queries that will
> insert/update/delete other tuples (usually, but not necessarily, in
> other tables).  Even if it worked, a rule would be a horribly
> inefficient way to handle modification of the about-to-be-inserted
> tuple, because (being an independent query) it'd have to scan the table
> to find the tuple you are talking about!
>
> The reason the additional queries are done before the original command
> is explained thus in the source code:
>
>      * The original query is appended last if not instead
>      * because update and delete rule actions might not do
>      * anything if they are invoked after the update or
>      * delete is performed. The command counter increment
>      * between the query execution makes the deleted (and
>      * maybe the updated) tuples disappear so the scans
>      * for them in the rule actions cannot find them.
>
> This seems to make sense for UPDATE/DELETE, but I wonder whether
> the ordering should be different for the INSERT case: perhaps it
> should be original-query-first in that case.
>

Thanks, Tom.  I was writing the Trigger section of my book the past few
days, and this helped me define when to use rules and when to use
triggers.


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