Horton

Re: [HACKERS] [PATCH] Generic type subscripting

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Dmitry Dolgov <9erthalion6@gmail.com>
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: 2020-12-02T20:02:44Z
Lists: pgsql-hackers 
Dmitry Dolgov <9erthalion6@gmail.com> writes:
>> 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:
>> ...
>> 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'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?

Hmm, no, I just built with --disable-cassert and otherwise my usual
development options.

I had experimented with some other variants of the test case,
where the repeated statement is

    a[i] := i;                -- about the same
    a[i] := a[i-1] + 1;       -- 7% slower
    a[i] := a[i-1] - a[i-2];  -- 15% slower

so it seems clear that the penalty is on the array fetch not array
assign side.  This isn't too surprising now that I think about it,
because plpgsql's array assignment code is untouched by the patch
(which is a large feature omission BTW: you still can't write
    jsonb['x'] := y;
in plpgsql).

			regards, tom lane

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.