Re: Eval expression R/O once time (src/backend/executor/execExpr.c)

Hi,

On 2021-09-21 15:09:11 -0300, Ranier Vilela wrote:
> Currently when determining where CoerceToDomainValue can be read,
> it evaluates every step in a loop.
> But, I think that the expression is immutable and should be solved only
> once.

What is immutable here?


> Otherwise the logic is wrong since by the rules of C, even though the
> variable is
> being initialized in the declaration, it still receives initialization at
> each repetition.
> What causes palloc running multiple times.
> 
> In other words:
> Datum   *domainval = NULL;
> 
> is the same:
> Datum   *domainval;
> domainval = NULL;

Obviously?


> Thoughts?

I don't see what this is supposed to achieve. The allocation of
domainval/domainnull happens on every loop iteration with/without your patch.

And it has to, the allocation intentionally is separate for each
constraint. As the comment even explicitly says:
                    /*
                     * Since value might be read multiple times, force to R/O
                     * - but only if it could be an expanded datum.
                     */
Greetings,

Andres Freund