On 24.08.2019 19:13, Pavel Stehule wrote:
so 24. 8. 2019 v 18:01 odesílatel David Fetter <da...@fetter.org
<mailto:da...@fetter.org>> napsal:
On Fri, Aug 23, 2019 at 11:10:28AM +0200, Pavel Stehule wrote:
> pá 23. 8. 2019 v 11:05 odesílatel Konstantin Knizhnik <
> k.knizh...@postgrespro.ru <mailto:k.knizh...@postgrespro.ru>>
napsal:
>
> >
> >
> > On 22.08.2019 18:56, Pavel Stehule wrote:
> >
> >
> >
> > čt 22. 8. 2019 v 17:51 odesílatel Konstantin Knizhnik <
> > k.knizh...@postgrespro.ru <mailto:k.knizh...@postgrespro.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.
No, it's lying to the RDBMS, so it's pilot error. The problem of
determining from the function itself whether it is in fact immutable
is, in general, equivalent to the Halting Problem, so no, we can't
figure it out. We do need to trust our users not to lie to us, and we
do not need to protect them from the consequences when they do.
I have not any problem with fixing this behave when there will be any
alternative.
I can imagine new special flag that can be used for STABLE functions,
that enforce one shot plans and can be optimized similar like
IMMUTABLE functions now - using result in planning time.
The users lie because they must - there is not a alternative. There is
not any other solution - and estimation errors related to a joins are
fundamental issue.
Pavel, I wonder if I can put my patch (with fix which performs this
optimization only for built-in functions) to commitfest or you prefer to
do it yourself in some other way and propose your own solution?
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company