Re: STABLE functions

Hi Tom,

On Fri, Apr 25, 2003 at 10:01:44AM -0400, Tom Lane wrote:
> > I thought that this function is only executed once when declared as
> > STABLE,

> You are mistaken.

> If you'd declared it IMMUTABLE, then it would get constant-folded, but
> that may or may not be suitable for your purpose.

The function in question is not IMMUTABLE :-( It depends on database
lookups but won't change its results within one transaction.

Quotation from the docs:

| STABLE indicates that within a single table scan the function will
| consistently return the same result for the same argument values, but
| that ist result could change across SQL statements.

So why is a

SELECT ... FROM table WHERE col = f(...)

(with a STABLE function f) not a "single table scan" or why does
PostgreSQL re-calculate the value of f() here for every row?

Thanks a lot,
Joachim

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