Josh Berkus wrote:
> Ludwig,
>
> > I tried converting the rule to function & trigger
> > and it reported that it used an Index scan given the
> > same INSERT statement.
> >
> > I was wondering why the rule used sequentail scan
> > and the equivalent function trigger used index scan.
>
> This is one of the documented differences between rules and triggers
> (at least, I've seen it in the documentation *somewhere*). I can't
> tell you "why", though ... hopefully Tom or Bruce will speak to that.
Neither is *that* one of the differences between rules and triggers, nor *should* something like that be
documented instead of fixed.
There might be a difference in the selectivity when using OLD/NEW Var nodes vs. $n parameters, and
therefore a different estimate by the optimizer. But in general rule generated queries are using indexes.
Of course, that requires up to date statistics. Are the tables ANALYZE'd properly?
> For myself, I use triggers for most things, and rules just for
> updatable views. Aside for index efficiency, rules are a Postgres-only
> thing, whereas several RDBMS's support triggers. Gives me marginally
> better portability.
As long as your queries only affect very few rows, you'll not notice much of a performance difference. Try
updating a couple thousand rows with one UPDATE. A trigger will fire a couple thousand times, where a rule does
allthe work in one additional query.
Jan
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