Horton

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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: David Rowley <dgrowleyml@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>, Nikita Glukhov <n.gluhov@postgrespro.ru>, Andrew Dunstan <andrew@dunslane.net>
Date: 2022-06-17T17:21:38Z
Lists: pgsql-hackers 
Hi,

On 2022-06-17 14:14:54 +1200, David Rowley wrote:
> 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:

What do you think about the approach prototyped in my patch to move the hash
FunctionCallInfo into the element_tab? With a tiny bit more work that should
reduce the amount of dereferincing over the state today, while also keeping
below the limit?

> 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.

"smaller step size"?

Greetings,

Andres Freund

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