Re: Why overhead of SPI is so large?

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 <mailto: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 <mailto: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?

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