Обсуждение: Quick Help

Hi guys,

Messing about with ADD COLUMN...

I'm not certain how to re-evaluate the default expression for each row?  How
do I do this?  I have access to raw_default and cooked_default it seems.

Thanks,

Chris

It works very similarly to the way that ALTER TABLE ... ADD CHECK ..
works, with the tuple update added in.

Anyway, it's something like the below:

- Lock relation
- Pull out tuple
- Evaluate cooked default expression using EvalExpr
- heap_modifytuple (shove datum that EvalExpr returns into column)
- simple_heapupdate
- Goto step 2 (pull out next tuple)



On Thu, 2002-12-05 at 16:24, Christopher Kings-Lynne wrote:
> Hi guys,
>
> Messing about with ADD COLUMN...
>
> I'm not certain how to re-evaluate the default expression for each row?  How
> do I do this?  I have access to raw_default and cooked_default it seems.
>
> Thanks,
>
> Chris
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster
--
Rod Taylor <rbt@rbt.ca>

PGP Key: http://www.rbt.ca/rbtpub.asc

"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
> Messing about with ADD COLUMN...

> I'm not certain how to re-evaluate the default expression for each
> row?

Basically you'll want to apply ExecEvalExpr (or perhaps
ExecEvalExprSwitchContext) to the "cooked" expression tree.
Look at the execution of per-row constraints in ADD CONSTRAINT;
should be very very similar.
        regards, tom lane