Changes to get_random_community_id: add filters for nsfw and private, use algorthm that doesn't scan the entire table #5267
Conversation
…thm that doesn't scan the entire table
dullbananas
requested review from
dessalines,
phiresky,
Nutomic and
SleeplessOne1917
as code owners
| CREATE INDEX idx_community_random_number ON community (random_number) INCLUDE (local, nsfw) | ||
| WHERE | ||
| NOT (deleted OR removed OR visibility = 'Private'); | ||
|
|
There was a problem hiding this comment.
why smallint? due to birthday paradox you have a high collision probability already with 2^(n/2) communities = 2^8 = 256 communities
There was a problem hiding this comment.
It uses less space, and collisions are fine because the random number is updated after the community is picked
There was a problem hiding this comment.
why would you update the number after picking?
There was a problem hiding this comment.
Its described here. I'm sure there are others ways of doing it, allowing collisions and updating the random number after picking, being one way to do it.
There was a problem hiding this comment.
right, i missed the last paragraph there. Still seems a bit awkward to me, especially having a read-only innocuous request cause a table write.
Why don't you just do select * from community where id = 1+(random() * (select max(id) from community))::int in a loop? The id of communities is already unique and uniformly distributed, with a fill rate of like 99% (so needing more than one/two queries is exceedingly unlikely) - no new column or modifying query or function needed.
There was a problem hiding this comment.
I guess that would make filtered (nswf/local) more difficult, mh.
| CREATE INDEX idx_community_random_number ON community (random_number) INCLUDE (local, nsfw) | ||
| WHERE | ||
| NOT (deleted OR removed OR visibility = 'Private'); | ||
|
|
There was a problem hiding this comment.
Its described here. I'm sure there are others ways of doing it, allowing collisions and updating the random number after picking, being one way to do it.
migrations/2024-12-18-200602_optimize_get_random_community/up.sql
Outdated
Show resolved
Hide resolved
No description provided.