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Re: Way to stop recursion?

From: Pierre-Frédéric Caillaud <lists(at)boutiquenumerique(dot)com>
To: pgsql-sql(at)postgresql(dot)org
Subject: Re: Way to stop recursion?
Date: 2004-11-26 22:21:51
Message-ID: opsh3e2ph5cq72hf@musicbox (view raw or flat)
Thread:
Lists: pgsql-sql
> You have to do this with a trigger.  The problem is that the rule is
> expanded inline like a macro, so you can't prevent the behaviour
> you're seeing.

	True, but you can get out of the hole in another way :

- Change the name of your table to "hidden_table"

- Create a view which is a duplicate of your table :
CREATE VIEW visible_table AS SELECT * FROM hidden_table;

-> Your application now accesses its data without realizing it goes  
through a view.

Now create a rule on this view, to make it update the real hidden_table.  
As the rule does not apply to hidden_table, it won't recurse.

Other solution (this similar to what Tom Lane proposed I think) :

Create a field common_id in your table, with
- an insert trigger which puts a SERIAL default value if there is no  
parent, or copies the parent's value if there is one
- an update trigger to copy the new parent's common_id whenever a child  
changes parent (if this ever occurs in your design)

Now create another table linking common_id to the 'common' value.

Create a view which joins the two, which emulates your current behaviour.
Create an ON UPDATE rule to the view which just changes one row in the  
link table.

If you do a lot of selects, solution #1 will be faster, if you do a lot of  
updates, #2 will win...

Just out of curiosity, what is this for ?


On Fri, 26 Nov 2004 16:34:48 -0500, Andrew Sullivan <ajs(at)crankycanuck(dot)ca>  
wrote:

> On Fri, Nov 26, 2004 at 01:03:38PM -0800, Jonathan Knopp wrote:
>> UPDATE rules work perfectly for what I need to do except I need them to
>> only run once, not try and recurse (which of course isn't allowedby
>> postgresql anyway). Triggers seem a less efficient way to do the same
>> thing, though I understand they would run recursively too. Here's the
>> table structure in question:
>
> You have to do this with a trigger.  The problem is that the rule is
> expanded inline like a macro, so you can't prevent the behaviour
> you're seeing.
>
> A
>



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