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: 2020-12-02T17:58:51Z
Lists: pgsql-hackers 
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 don't think this can be considered RFC until the performance
issue is addressed.

			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.