On Wed, 2023-02-08 at 16:59 +0800, tender wang wrote:
> In evaluate_function(), I find codes as shown below:
>
> /*
> * Ordinarily we are only allowed to simplify immutable functions. But for
> * purposes of estimation, we consider it okay to simplify functions that
> * are merely stable; the risk that the result might change from planning
> * time to execution time is worth taking in preference to not being able
> * to estimate the value at all.
> */
> if (funcform->provolatile == PROVOLATILE_IMMUTABLE)
> /* okay */ ;
> else if (context->estimate && funcform->provolatile == PROVOLATILE_STABLE)
> /* okay */ ;
> else
> return NULL;
>
> The codes say that stable function can not be simplified here(e.g. planning phase).
> I want to know the reason why stable function can not be simplified in planning phase.
> Maybe show me a example that it will be incorrect for a query if simplify stable function in
> planning phases.
Query planning and query execution can happen at different times and using
different snapshots, so the result of a stable function can change in the
meantime. Think of prepared statements using a generic plan.
Yours,
Laurenz Albe