Re: Postgres memoizing inner loop of join when outer loop join key is guaranteed unique?

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From: Ron Johnson <[email protected]>
To: Jacob Jackson <[email protected]>
Cc: pgsql-generallists.postgresql.org <[email protected]>
Subject: Re: Postgres memoizing inner loop of join when outer loop join key is guaranteed unique?
Date: Wed, 29 Oct 2025 17:48:23 -0400
Message-ID: <CANzqJaDrojumO16bBSWVDg65i5pQM0mdsojfrPO2UyP3t3mUkg@mail.gmail.com> (raw)
In-Reply-To: <CAAiQw3ysEm38FA7ZuMKRnui1X17sscjRWqEV8Gx4tz3zPRnxiw@mail.gmail.com>
References: <CAAiQw3yBPrCw6ZLeTwVS4QhKDWgJkmmp9LnGPdodxeQmn=kqVg@mail.gmail.com>
	<CANzqJaC3H9GaqQTKUS75_TTJGYjuGyT+jNf+VLG7a9vy_GwhRw@mail.gmail.com>
	<CAAiQw3ysEm38FA7ZuMKRnui1X17sscjRWqEV8Gx4tz3zPRnxiw@mail.gmail.com>

You're running slightly different queries:
enable_memoize=on: Index Cond: ("user" = '0'::bigint)
enable_memoize=off: Index Cond: ("user" = '3477145805513'::bigint)

All buffer counts look to be the same on both, and 514 is just 1.5% smaller
than 522.  That looks like statistical noise to me.


On Wed, Oct 29, 2025 at 5:34 PM Jacob Jackson <[email protected]>
wrote:

> I was curious to see whether there was any reason I wasn't seeing for
> Postgres to decide the memoized version was lower cost and try to memoize
> these operations.
>
> On Wed, Oct 29, 2025 at 3:20 PM Ron Johnson <[email protected]>
> wrote:
>
>> What's the actual problem?  Does enable_memoize=on return incorrect
>> results?
>>
>> Because a 45 microsecond (yes, 45 microseconds: 0.138 milliseconds = 138
>> microseconds; same for the others) slowdown isn't something I'd get too
>> worked up about.
>>
>> On Wed, Oct 29, 2025 at 2:29 PM Jacob Jackson <[email protected]>
>> wrote:
>>
>>> Hello. I was looking at some query plans recently and noticed something
>>> that didn't make sense. I have a query that joins a table of questions with
>>> results for each question (using a table with a composite primary key of
>>> question id and a user id), filtered by user id. The question IDs and the
>>> combined question-userIds are guaranteed unique due to being primary keys,
>>> and yet Postgres still memoizes the inner loop results. Any ideas why? Is
>>> this just a failure of the query planner (I would be happy to explore
>>> creating a PR), did I not properly guarantee uniqueness, or is there
>>> another reason for memoization? The memoized version is consistently
>>> slightly slower in my testing, despite the calculated cost being lower.
>>> Here are the query plans:
>>>
>>> enable_memoize=on:
>>>
>>> EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM questions LEFT JOIN
>>> "QuestionUserStatus" ON questions.id = "QuestionUserStatus".question
>>> WHERE "QuestionUserStatus".user = 0;
>>>
>>> Nested Loop  (cost=0.71..514.09 rows=277 width=1381) (actual
>>> time=0.021..0.520 rows=231 loops=1)
>>> ├ Buffers: shared hit=859
>>> ├ Index Scan using "QuestionUserStatus_user_question_pk" on
>>> "QuestionUserStatus"  (cost=0.42..178.88 rows=277 width=18) (actual
>>> time=0.014..0.114 rows=231 loops=1)
>>> │ ├ Index Cond: ("user" = '0'::bigint)
>>> │ └ Buffers: shared hit=166
>>> └ Memoize  (cost=0.29..1.25 rows=1 width=1363) (actual time=0.001..0.001
>>> rows=1 loops=231)
>>>   ├ Cache Key: "QuestionUserStatus".question
>>>   ├ Cache Mode: logical
>>>   ├ Hits: 0  Misses: 231  Evictions: 0  Overflows: 0  Memory Usage: 320kB
>>>   ├ Buffers: shared hit=693
>>>   └ Index Scan using questions_pkey on questions  (cost=0.28..1.24
>>> rows=1 width=1363) (actual time=0.001..0.001 rows=1 loops=231)
>>>     ├ Index Cond: (id = "QuestionUserStatus".question)
>>>     └ Buffers: shared hit=693
>>> Planning:
>>> └ Buffers: shared hit=6
>>> Planning Time: 0.183 ms
>>> Execution Time: 0.548 ms
>>>
>>> enable_memoize=off:
>>>
>>> Nested Loop  (cost=0.70..521.98 rows=277 width=1381) (actual
>>> time=0.018..0.421 rows=231 loops=1)
>>> ├ Buffers: shared hit=859
>>> ├ Index Scan using "QuestionUserStatus_user_question_pk" on
>>> "QuestionUserStatus"  (cost=0.42..178.88 rows=277 width=18) (actual
>>> time=0.014..0.099 rows=231 loops=1)
>>> │ ├ Index Cond: ("user" = '3477145805513'::bigint)
>>> │ └ Buffers: shared hit=166
>>> └ Index Scan using questions_pkey on questions  (cost=0.28..1.24 rows=1
>>> width=1363) (actual time=0.001..0.001 rows=1 loops=231)
>>>   ├ Index Cond: (id = "QuestionUserStatus".question)
>>>   └ Buffers: shared hit=693
>>> Planning:
>>> └ Buffers: shared hit=6
>>> Planning Time: 0.197 ms
>>> Execution Time: 0.444 ms
>>>
>>> Thanks for the help,
>>> Jacob
>>>
>>
-- 
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> lobster!

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