Thread

1. change_varattnos_of_a_node versus whole-row Vars

   Tom Lane <tgl@sss.pgh.pa.us> — 2012-06-29T17:20:02Z

   change_varattnos_of_a_node(), which is used to adjust expressions
   referencing a parent relation or LIKE source relation to refer to the
   child relation, ignores whole-row Vars (those with attnum zero).  
   Thus, after processing the expression, the whole-row Var still claims
   to have the rowtype (vartype) of the parent rel.  This is utterly wrong
   in the LIKE case, as pointed out in
   http://archives.postgresql.org/pgsql-general/2012-06/msg00674.php
   and AFAICS it is wrong in the inheritance case too, since there's
   no guarantee that the parent and child rowtypes are binary-equivalent.
   
   My inclination, especially in the back branches, is to just throw error
   if we find a whole-row Var.  ISTM the entire point of CREATE TABLE LIKE
   is that the two tables' rowtypes might diverge after the child is
   created, so it makes no sense to do anything that would presuppose that
   an expression involving the parent's rowtype will remain sensible for
   the child.  The only caller not associated with CREATE TABLE LIKE is
   MergeAttributes, which is trying to adjust CHECK constraints for a
   parent rel to apply to a new inheritance child.  In that case it would
   be sensible to expect a "parent.*" reference to be valid for the child
   (though a ConvertRowtypeExpr would still need to be thrown in to make
   this work).  However, it's hard to implement because the Var would need
   to be modified to contain the rowtype OID for the child relation, which
   has not been assigned at this point.  In principle we could rearrange
   CREATE TABLE processing enough so that inherited CHECK constraints
   aren't adjusted till after we know the rowtype OID, but I'm not
   personally volunteering to do that.
   
   There are a bunch of other things I don't like about
   change_varattnos_of_a_node, starting with the name, but those gripes are
   in the nature of cosmetics or inadequate error checking and wouldn't
   have any user-visible impact if changed.
   
   Any thoughts on the topic?
   
   			regards, tom lane
   
   

2. Re: change_varattnos_of_a_node versus whole-row Vars

   Tom Lane <tgl@sss.pgh.pa.us> — 2012-06-29T22:54:11Z

   I wrote:
   > change_varattnos_of_a_node(), which is used to adjust expressions
   > referencing a parent relation or LIKE source relation to refer to the
   > child relation, ignores whole-row Vars (those with attnum zero).  
   > ...
   > My inclination, especially in the back branches, is to just throw error
   > if we find a whole-row Var.
   > ...
   > There are a bunch of other things I don't like about
   > change_varattnos_of_a_node, starting with the name, but those gripes are
   > in the nature of cosmetics or inadequate error checking and wouldn't
   > have any user-visible impact if changed.
   
   Attached is a draft patch that adds error reports for whole-row Vars
   and replaces change_varattnos_of_a_node with a more robust function.
   The latter makes it kind of a large patch, but it's difficult to make
   it a whole lot simpler unless we are willing to have the error messages
   say only "cannot convert whole-row table reference", without any
   indication of where the problem is.  That seems a tad unfriendly to me.
   
   Comments?
   
   			regards, tom lane