Re: Truncate if exists

On 10/09/2012 04:06 PM, Tom Lane wrote:
> Second, to my mind the point of a multi-table TRUNCATE is to ensure that
> all the referenced tables get reset to empty *together*.  With something
> like this, you'd have no such guarantee.  Consider a timeline like this:
> 
> 	Session 1			Session 2
> 
> 	TRUNCATE IF EXISTS a, b, c;
> 	... finds c doesn't exist ...
> 	... working on a and b ...
> 					CREATE TABLE c ( ... );
> 					INSERT INTO c ...;
> 	... commits ...
> 
> Now we have a, b, and c, but c isn't empty, violating the expectations
> of session 1.  So even if there's a use-case for IF EXISTS on a single
> table, I think it's very very dubious to allow it in multi-table
> commands.

Hi,

I've to say that I don't understand your timeline :

- If c exist in Session 1, CREATE TABLE in Session 2 can't be done,
neither INSERT
- If c doesn't exists in Session 1, it will be ignored, then, Session 2
work fine.

In any case, TRUNCATE is sent before INSERT, but it can't lock an object
which still not exists.

I understand that people don't want TRUNCATE IF EXISTS, but, in my point
of view, even if TRUNCATE is not a DDL, it's the same use-case as DROP
TABLE IF EXISTS.

Regards,

-- 
Sébastien Lardière
PostgreSQL DBA Team Manager
Hi-Media