Robert Haas <robertmhaas@gmail.com> writes:
> On Sun, Nov 22, 2009 at 6:51 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> CREATE IF NOT EXISTS has been proposed and rejected before, more than
>> once. �Please see the archives.
> Search for CINE to find the discussions. This is a good place to start:
> http://archives.postgresql.org/pgsql-hackers/2009-05/msg00252.php
> Despite Tom's assertions to the contrary, I am unable to find a clear
> consensus against this feature in the archives,
I think you didn't look back far enough --- that issue was settled years
ago. IIRC the killer argument is that after CINE you do not know the
state of the object: it exists, yes, but what properties has it got?
If it already existed then it's still got its old definition, which
might or might not be what you're expecting.
CREATE OR REPLACE has got far safer semantics from the viewpoint of a
script that wants to bull through without having any actual error
handling (which is more or less the scenario we are arguing here, no?)
After successful execution of the command you know exactly what
properties the object has got.
Whether it would be sensible to have CREATE OR REPLACE semantics for a
language is something I'm not very clear on. It seems like changing any
of the properties of a pg_language entry could be rather fatal from the
viewpoint of an existing function using the language.
[ thinks for awhile... ] Actually, CREATE LANGUAGE is unique among
creation commands in that the common cases have no parameters, at least
not since we added pg_pltemplate. So you could imagine defining CINE
for a language as disallowing any parameters and having these semantics:
* language not present -> create from template
* language present, matches template -> OK, do nothing
* language present, does not match template -> report error
This would meet the objection of not being sure what the state is
after successful execution of the command. It doesn't scale to any
other object type, but is it worth doing for this one type?
regards, tom lane
