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From: Jacob Jackson <[email protected]>
To: [email protected]
Subject: Postgres memoizing inner loop of join when outer loop join key is guaranteed unique?
Date: Wed, 29 Oct 2025 14:28:40 -0400
Message-ID: <CAAiQw3yBPrCw6ZLeTwVS4QhKDWgJkmmp9LnGPdodxeQmn=kqVg@mail.gmail.com> (raw)

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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