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

The performance/cost difference persists with identical keys. Why would the
planner for the memorized query use different statistics/calculations in
the first place?

On Wed, Oct 29, 2025 at 5:48 PM Ron Johnson <ronljohnsonjr(at)gmail(dot)com> wrote:

> 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 <jej(dot)jackson(dot)08(at)gmail(dot)com>
> 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 <ronljohnsonjr(at)gmail(dot)com>
>> 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 <jej(dot)jackson(dot)08(at)gmail(dot)com>
>>> 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
>>>>
>>>
> --
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> Don't boil me, I'm still alive.
> <Redacted> lobster!
>