Horton

Re: [HACKERS] [PATCH] Generic type subscripting

Dmitry Dolgov <9erthalion6@gmail.com>

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From: Dmitry Dolgov <9erthalion6@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alexander Korotkov <aekorotkov@gmail.com>, Pavel Stehule <pavel.stehule@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, David Steele <david@pgmasters.net>, Nikita Glukhov <n.gluhov@postgrespro.ru>, Alvaro Herrera <alvherre@2ndquadrant.com>, David Fetter <david@fetter.org>, Thomas Munro <thomas.munro@gmail.com>, Oleksandr Shulgin <oleksandr.shulgin@zalando.de>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, Michael Paquier <michael.paquier@gmail.com>, Oleg Bartunov <obartunov@gmail.com>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-12-02T19:18:08Z
Lists: pgsql-hackers 
> On Wed, Dec 02, 2020 at 12:58:51PM -0500, Tom Lane wrote:
> So ... one of the things that's been worrying me about this patch
> from day one is whether it would create a noticeable performance
> penalty for existing use-cases.  I did a small amount of experimentation
> about that with the v35 patchset, and it didn't take long at all to
> find that this:
>
> --- cut ---
> create or replace function arraytest(n int) returns void as
> $$
> declare
>   a int[];
> begin
>   a := array[1, 1];
>   for i in 3..n loop
>     a[i] := a[i-1] - a[i-2];
>   end loop;
> end;
> $$
> language plpgsql stable;
>
> \timing on
>
> select arraytest(10000000);
> --- cut ---
>
> is about 15% slower with the patch than with HEAD.  I'm not sure
> what an acceptable penalty might be, but 15% is certainly not it.
>
> I'm also not quite sure where the cost is going.  It looks like
> 0001+0002 aren't doing much to the executor except introducing
> one level of subroutine call, which doesn't seem like it'd account
> for that.

I've tried to reproduce that, but get ~2-4% slowdown (with a pinned
backend, no turbo etc). Are there any special steps I've probably
missed? At the same time, I remember had conducted this sort of tests
before when you and others raised the performance degradation question
and the main part of the patch was already more or less stable. From
what I remember the numbers back then were also rather small.

Commits

  1. Throw error when assigning jsonb scalar instead of a composite object

  2. Filling array gaps during jsonb subscripting

  3. Implementation of subscripting for jsonb

  4. Allow ALTER TYPE to update an existing type's typsubscript value.

  5. Allow subscripting of hstore values.

  6. Support subscripting of arbitrary types, not only arrays.

  7. jit: Reference function pointer types via llvmjit_types.c.

  8. Teach contain_leaked_vars that assignment SubscriptingRefs are leaky.

  9. jit: Correct parameter type for generated expression evaluation functions.

  10. Renaming for new subscripting mechanism

  11. Fix assertion failure for SSL connections.

  12. Teach eval_const_expressions() to handle some more cases.