On Mon, Feb 5, 2024 at 7:23 AM Sean v <s...@vanmulligen.ca> wrote: > This is related to a question I asked on dbs.stackexchange.com: > https://dba.stackexchange.com/questions/335501/why-doesnt-postgres-apply-limit-on-groups-when-retrieving-n-results-per-group > > But to reiterate - I have a query like this: > > SELECT "orders".* > > FROM "orders" > > WHERE (user_id IN ?, ?, ?) > > ORDER BY "orders"."created_at" LIMIT 50 > [snip]
> So my question is twofold: > - why doesn't Postgres use the composite index, and then retrieve only the > minimum necessary amount of rows (50 per user) using the query I posted > above? > > But your query *does not* list the first 50 rows *per user*. It only returns the first 50 rows of: SELECT "orders".* FROM "orders" WHERE (user_id IN ?, ?, ?) ORDER BY "orders"."created_at" Who knows which users are going to be in that list???
