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From: Chao Li <[email protected]>
To: David Rowley <[email protected]>
Cc: John Naylor <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Peter Geoghegan <[email protected]>
Subject: Re: tuple radix sort
Date: Thu, 30 Oct 2025 13:43:34 +0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAApHDvpGd-cHSr5OZ7_JpoTSWDBZAvAkarbkJPd0J3FW9zvFGw@mail.gmail.com>
References: <CANWCAZYzx7a7E9AY16Jt_U3+GVKDADfgApZ-42SYNiig8dTnFA@mail.gmail.com>
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	<CANWCAZbrKH-sDvzqb5z8BEofpXyXuN42UCRzoQ8wT3CiqPATFg@mail.gmail.com>
	<[email protected]>
	<CAApHDvpGd-cHSr5OZ7_JpoTSWDBZAvAkarbkJPd0J3FW9zvFGw@mail.gmail.com>



> On Oct 30, 2025, at 13:01, David Rowley <[email protected]> wrote:
> 
> On Thu, 30 Oct 2025 at 16:46, Chao Li <[email protected]> wrote:
>>> On Oct 30, 2025, at 11:40, John Naylor <[email protected]> wrote:
>>> Are you by chance running with asserts on? It's happened before, so I
>>> have to make sure. That makes a big difference here because I disabled
>>> diversion thresholds in assert builds so that regression tests (few
>>> cases with large inputs) cover the paths I want, in addition to my
>>> running a standalone stress test.
>> 
>> Yes, assert is always enabled in my sandbox. I can disable assert and rerun the test later.
> 
> Never expect anything meaningful to come from running performance
> tests on Assert builds. You should always be rebuilding without
> Asserts before doing performance tests.
> 

Sure, good to learn. Actually I am very new to PG development, so any guidance is greatly appreciated.

I just made a distclean, then configure without any parameter. Now, the overall execution time reduced ~10% than with asserts. With the low cardinality data, off and on are very close:

```
evantest=# set wip_radix_sort = 'off';
Time: 0.206 ms
evantest=# select * from test_multi order by category, name;
Time: 5070.277 ms (00:05.070)
evantest=# select * from test_multi order by category, name;
Time: 5158.748 ms (00:05.159)
evantest=# select * from test_multi order by category, name;
Time: 5072.708 ms (00:05.073)

evantest=# set wip_radix_sort = 'on';
Time: 0.177 ms
evantest=# select * from test_multi order by category, name;
Time: 4992.516 ms (00:04.993)
evantest=# select * from test_multi order by category, name;
Time: 5145.361 ms (00:05.145)
evantest=# select * from test_multi order by category, name;
Time: 5101.800 ms (00:05.102)

evantest=# \o
evantest=# show work_mem;
 work_mem
----------
 1GB
(1 row)

Time: 0.186 ms
evantest=# explain select * from test_multi order by category, name;
                                QUERY PLAN
---------------------------------------------------------------------------
 Sort  (cost=122003.84..124503.84 rows=1000000 width=69)
   Sort Key: category, name
   ->  Seq Scan on test_multi  (cost=0.00..22346.00 rows=1000000 width=69)
(3 rows)
```

And with high cardinality test data, on has a big win:
```
evantest=# set wip_radix_sort = 'off';
Time: 0.174 ms
evantest=# select * from test_multi order by category, name;
Time: 353.702 ms
evantest=# select * from test_multi order by category, name;
Time: 375.549 ms
evantest=# select * from test_multi order by category, name;
Time: 367.967 ms
evantest=# set wip_radix_sort = 'on';
Time: 0.147 ms
evantest=# select * from test_multi order by category, name;
Time: 279.537 ms
evantest=# select * from test_multi order by category, name;
Time: 278.114 ms
evantest=# select * from test_multi order by category, name;
Time: 284.273 ms
```

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/

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