Re: Truncate if exists

Simon Riggs <simon@2ndQuadrant.com> writes:
> On 9 October 2012 09:33, Sbastien Lardire <slardiere@hi-media.com> wrote:
>> With the help of Cdric, here's a patch changing the TRUNCATE TABLE
>> command, adding the IF EXISTS option to allow the presence in the list
>> of tables of a missing or invisible table.

> Will apply in 48 hours barring objections.

I object: this doesn't deserve to be fast-tracked like that with no
thought about whether the semantics are actually useful or sensible.

For starters, the use-case hasn't been explained to my satisfaction.
In what situation is it actually helpful to TRUNCATE a table that's
not there yet?  Aren't you going to have to do a CREATE IF NOT EXISTS
to keep from failing later in the script?  If so, why not just do that
first?

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.

			regards, tom lane