Re: Why cann't simplify stable function in planning phase?

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