Re: Why overhead of SPI is so large?

pá 23. 8. 2019 v 11:05 odesílatel Konstantin Knizhnik <
k(dot)knizhnik(at)postgrespro(dot)ru> napsal:

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> On 22.08.2019 18:56, Pavel Stehule wrote:
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> čt 22. 8. 2019 v 17:51 odesílatel Konstantin Knizhnik <
> k(dot)knizhnik(at)postgrespro(dot)ru> napsal:
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>> 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.
>>
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> 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.
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> I wonder if the following simple patch is correct?
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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 your patch is good enough for benchmarking.

Pavel

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