Re: Why overhead of SPI is so large?

pá 23. 8. 2019 v 13:21 odesílatel Konstantin Knizhnik <
k(dot)knizhnik(at)postgrespro(dot)ru> napsal:

>
>
> On 23.08.2019 12:10, Pavel Stehule wrote:
>
>
>
> pá 23. 8. 2019 v 11:05 odesílatel Konstantin Knizhnik <
> k(dot)knizhnik(at)postgrespro(dot)ru> napsal:
>
>>
>>
>> On 22.08.2019 18:56, Pavel Stehule wrote:
>>
>>
>>
>> čt 22. 8. 2019 v 17:51 odesílatel Konstantin Knizhnik <
>> k(dot)knizhnik(at)postgrespro(dot)ru> napsal:
>>
>>> Some more information...
>>> First of all I found out that marking PL/pgSQL function as immutable
>>> significantly increase speed of its execution:
>>> 19808 ms vs. 27594. It happens because exec_eval_simple_expr is taken
>>> snapshot if function is volatile (default).
>>> I wonder if PL/pgSQL compiler can detect that evaluated expression
>>> itself is actually immutable and there is no need to take snapshot
>>> for each invocation of this function. Also I have tried yet another PL
>>> language - JavaScript, which is now new outsider, despite to the fact that
>>> v8 JIT compiler is very good.
>>>
>>
>> I have a plan to do some work in this direction. Snapshot is not
>> necessary for almost buildin functions. If expr calls only buildin
>> functions, then probably can be called without snapshot and without any
>> work with plan cache.
>>
>>
>> I wonder if the following simple patch is correct?
>>
>
> You cannot to believe to user defined functions so immutable flag is
> correct. Only buildin functions are 100% correct.
>
> CREATE OR REPLACE FUNCTION foo()
> RETURNS int AS $$
> SELECT count(*) FROM pg_class;
> $$ LANGUAGE sql IMMUTABLE;
>
> is working.
>
>
> But such definition of the function contradicts IMMUTABLE contract,
> doesn't it?
> If creator of the function incorrectly classify it, then usage of such
> function can cause incorrect behavior.
> For example, if function is marked as "parallel safe" but actually it is
> not parallel safe, then using it in parallel plan may cause incorrect
> results.
> But it is a reason for disabling parallel plans for all user defined
> functions, isn't it?
>

In reality it is not IMMUTABLE function. On second hand, there are lot of
application that depends on this behave.

It is well know trick how to reduce estimation errors related to JOINs.
When immutable function has constant parameters, then it is evaluated in
planning time.

So sometimes was necessary to use

SELECT ... FROM tab WHERE foreign_key = immutable_function('constant
parameter')

instead JOIN.

It is ugly, but it is working perfectly. I think so until we will have
multi table statistics, this behave should be available in Postgres.

Sure, this function should not be used for functional indexes.

>
> Also nothing terrible will happen in any case. If expression is calling
> function which is marked is immutable but actually is not, then we can
> just get old (deteriorated)
> result of expression. Right now, if caller function (one containing
> evaluated expression) is marked as non-volatile, then snapshot is also not
> taken.
> So if such function expression is calling foo() function as declared
> above, then results will be also incorrect.
> So I do not think some principle difference here and do not understand why
> we should not believe user (function creator) only in this case.
>

>
>
> --
> Konstantin Knizhnik
> Postgres Professional: http://www.postgrespro.com
> The Russian Postgres Company
>
>