Re: rule on update

Gerhard Pfeiffer wrote:
> Hallo,
>
> I have a table with a timestamp-field, which should contain the date, when
> the row was last-updated.
> To maintain it, I tried to create a rule:
>
> CREATE RULE rule_entries_update_self
> AS ON UPDATE TO entries where new.ent_udate=old.ent_udate DO INSTEAD
> UPDATE entries SET ent_id=new.ent_id, ent_id_parent=new.ent_id_parent,
> ent_name=new.ent_name, ent_desc=new.ent_desc, ent_type=new.ent_type,
> ent_uname=CURRENT_USER, ent_udate=CURRENT_TIMESTAMP WHERE ent_id=new.ent_id;
>
> But then, doing an update it get's me:
> ERROR: query rewritten 10 times, may contain cycles
>
> It shouldn't show me this error, because the update invoked by the rule
> doesn't fulfill the condition (new.ent_udate=old.ent_udate).
>
> I just can't see, what's wrong here.
> (it's postgresql 7.1.3)

First you cannot guarantee that there will never be any rows
holding a future timestamp in ent_udate! So how can you say
that after rewriting the first time,
new.ent_udate=old.ent_udate will never be true?

Second, the parsetree the rule system is working on is too
abstract to allow such conclusions. Read chapter 8 of the
programmers manual to understand how the rule system works in
detail.

What you "want" to use here is a BEFORE UPDATE trigger that
put's the current timestamp into NEW.

Jan

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