Re: Postgres memoizing inner loop of join when outer loop join key is guaranteed unique? - Mailing list pgsql-general

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@gmail.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.jackson.08@gmail.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