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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"Gerhard Pfeiffer" <gp@bnbt.de> writes:

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).

Rules are macros rewritten at planning time, not at execution time,
so you cannot escape the rewrite cycle with an execution-time condition.

You will probably have better luck doing what you want to do with
a trigger.

regards, tom lane