Re: Backend misfeasance for DEFAULT NULL

On Sun, 2007-10-28 at 12:44 -0400, Tom Lane wrote:
> While poking at the complaint reported here:
> http://archives.postgresql.org/pgsql-general/2007-10/msg01484.php
> I realized that there is a related issue for null defaults.  Consider
> 
>     create table p (f1 int default 0);
>     create table c (f1 int);
>     alter table c inherit p;

Seems more like an unwanted looseness in the meaning of an ALTER
TABLE .. INHERIT to me. I'd prefer it if we added some extra clauses to
ALTER TABLE:

[ { INCLUDING | EXCLUDING } { DEFAULTS | CONSTRAINTS | INDEXES } ]

> At this point, c.f1 has no default, or NULL default if you prefer.
> However, pg_dump dumps this configuration as
> 
>     create table p (f1 int default 0);
>     create table c (f1 int) inherits (p);
> 
> so after a reload c.f1 will have default 0 because it'll inherit that
> from p.
> 
> I tried to fix this by having pg_dump insert an explicit DEFAULT NULL
> clause for c.f1, which turned out to be not too hard, but on testing
> it did nothing at all --- c.f1 still reloaded with default 0!
> 
> Poking into it, I find that it seems to be another case of the lesson
> we should have learned some time ago: embedding semantic knowledge in
> gram.y is usually a Bad Idea.  gram.y "knows" that it can throw away
> DEFAULT NULL --- see the exprIsNullConstant() uses therein.  So the
> clause never makes it to the place in tablecmds.c where we consider
> whether to adopt inherited defaults or not.
> 
> ISTM this is a backend bug: if I tell it DEFAULT NULL, by golly I
> should get DEFAULT NULL. 

Agreed.

--  Simon Riggs 2ndQuadrant  http://www.2ndQuadrant.com