Re: Truncate if exists

Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On 9 October 2012 09:33, Sbastien Lardire <slardiere(at)hi-media(dot)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