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
