Horton

Re: PG 15 (and to a smaller degree 14) regression due to ExprEvalStep size

James Coleman <jtc331@gmail.com>

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From: James Coleman <jtc331@gmail.com>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, PostgreSQL-development <pgsql-hackers@postgresql.org>, Nikita Glukhov <n.gluhov@postgrespro.ru>, Andrew Dunstan <andrew@dunslane.net>
Date: 2022-06-17T12:33:29Z
Lists: pgsql-hackers 
On Thu, Jun 16, 2022 at 10:15 PM David Rowley <dgrowleyml@gmail.com> wrote:
>
> On Fri, 17 Jun 2022 at 11:31, Andres Freund <andres@anarazel.de> wrote:
> > hashedscalararrayop/EEOP_HASHED_SCALARARRAYOP is 64 bytes, even though the
> > limit is 40 bytes.
>
> > commit 50e17ad281b8d1c1b410c9833955bc80fbad4078
> > Author: David Rowley <drowley@postgresql.org>
> > Date:   2021-04-08 23:51:22 +1200
> >
> >     Speedup ScalarArrayOpExpr evaluation
>
> I've put together the attached patch which removes 4 fields from the
> hashedscalararrayop portion of the struct which, once the JSON part is
> fixed, will put sizeof(ExprEvalStep) back down to 64 bytes again.
>
> The attached patch causes some extra pointer dereferencing to perform
> a hashed saop step, so I tested the performance on f4fb45d15 (prior to
> the JSON patch that pushed the sizeof(ExprEvalStep) up further. I
> found:
>
> setup:
> create table a (a int);
> insert into a select x from generate_series(1000000,2000000) x;
>
> bench.sql
> select * from a where a in(1,2,3,4,5,6,7,8,9,10);
>
> f4fb45d15 + reduce_sizeof_hashedsaop_ExprEvalStep.patch
> drowley@amd3990x:~$ pgbench -n -f bench.sql -T 60 -M prepared postgres
> tps = 44.841851 (without initial connection time)
> tps = 44.986472 (without initial connection time)
> tps = 44.944315 (without initial connection time)
>
> f4fb45d15
> drowley@amd3990x:~$ pgbench -n -f bench.sql -T 60 -M prepared postgres
> tps = 44.446127 (without initial connection time)
> tps = 44.614134 (without initial connection time)
> tps = 44.895011 (without initial connection time)
>
> (Patched is ~0.61% faster here)
>
> So, there appears to be no performance regression due to the extra
> indirection. There's maybe even some gains due to the smaller step
> size.

I didn't see that comment when working on this (it's quite a long
unioned struct; I concur on adding an assert to catch it).

This patch looks very reasonable to me though.

James Coleman

Commits

  1. JSON_TABLE: Add support for NESTED paths and columns

  2. Add basic JSON_TABLE() functionality

  3. Add SQL/JSON query functions

  4. Add soft error handling to some expression nodes

  5. Adjust populate_record_field() to handle errors softly

  6. Refactor code used by jsonpath executor to fetch variables

  7. Add more SQL/JSON constructor functions

  8. SQL/JSON: support the IS JSON predicate

  9. SQL/JSON: add standard JSON constructor functions

  10. Add static assertion ensuring sizeof(ExprEvalStep) <= 64 bytes

  11. Remove size increase in ExprEvalStep caused by hashed saops

  12. pgstat: reduce timer overhead by leaving timer running.

  13. expression eval: Fix EEOP_JSON_CONSTRUCTOR and EEOP_JSONEXPR size.

  14. SQL/JSON query functions

  15. Speedup ScalarArrayOpExpr evaluation