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From: Adrian Klaver <adrian.klaver@gmail.com>
To: pgsql-sql@postgresql.org
Subject: Re: on insert rule with default value
Date: Wed, 22 Feb 2012 07:32:10 -0800
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Cc: Ron Peterson <rpeterso@mtholyoke.edu>
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On Wednesday, February 22, 2012 5:52:39 am Ron Peterson wrote:
> 2012-02-21_15:51:30-0500 Ron Peterson <rpeterso@mtholyoke.edu>:
> > My rule below does not insert the the same uuid value into the test_log
> > table as is created in the test table when I insert a new value.  I know
> > I've worked through this before, but I'm not remembering why this is.
> > What's a right way to do this?
> 
> Obviously I can use a trigger function.  I'm mostly wondering if there
> are any tricks to accomplishing this with rules (I like the simple
> syntax).  I suspect the problem here is that 'new' on insert refers to
> the function used to calculate the new value, /not/ the new value that
> is actually inserted into the table.  There are probably reasons for
> that; but it would seem nicer to refer to the actual new table value
> rather than the expression used to calculate it.  My 2c.


The simplest explanation I could find is here:

http://www.postgresql.org/docs/9.0/interactive/querytree.html
"the target list...

For INSERT commands, the target list describes the new rows that should go into 
the result relation. It consists of the expressions in the VALUES clause or the 
ones from the SELECT clause in INSERT ... SELECT. The first step of the rewrite 
process adds target list entries for any columns that were not assigned to by 
the original command but have defaults. Any remaining columns (with neither a 
given value nor a default) will be filled in by the planner with a constant null 
expression.

"

If you want all the gory details read through section 37, in particular 37.3 :) 

The above is why I use triggers now. It is a lot easier to follow the logic in a 
trigger than in a rule.


> 
> -Ron-


-- 
Adrian Klaver
adrian.klaver@gmail.com