On 2/8/23 09:59, tender wang wrote:
> Hi hackers,
> 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.
>
A function is "stable" only within a particular execution - if you run a
query with a stable function twice, the function is allowed to return
different results.
If you consider parse analysis / planning as a separate query, this
explains why we can't simply evaluate the function in parse analysis and
then use the value in actual execution. See analyze_requires_snapshot()
references in postgres.c.
Note: To be precise this is not about "executions" but about snapshots,
and we could probably simplify the function call with isolation levels
that maintain a single snapshot (e.g. REPEATABLE READ). But we don't.
regards
--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company