Remove first two unused indexes

[neekolas]

tl;dr

We have a bunch of gigantic Postgres indexes that are completely unused. I am dropping the first three that feel uncontroversial.

Some data

These are our current database indexes

By running this SQL, I got the list of when they were last read:

SELECT relname , indexrelname , idx_scan , idx_tup_read , idx_tup_fetch
FROM pg_stat_user_indexes
WHERE schemaname = 'public' and relname = 'message';

Which tells me that this index has never been used in a query.

messageindex is used as a primary key so we can't kill it.

Next up

There are three more indexes that have been read in the past, but when I re-run the query above the values don't change or change very little.

message_recvts_shouldexpire_idx gets used extremely infrequently, as it has largely been replaced with message_recvts_shouldexpiretrue_idx which gets used a lot. But I'd like to understand why the delta isn't 0.

message_contenttopic_idx is also quite infrequently read. Because it shares a prefix with message_ctopic_sts_id_idx, I believe deleting it is safe and the other index could be used. But I'm also hesitant there.

message_sort_idx has basically never been used and is safe to delete.

[neekolas]

• Remove first two unused indexes #376 👈
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

Join @neekolas and the rest of your teammates on Graphite

[richardhuaaa]

Without CONCURRENTLY, does the server start up extremely slowly?

[neekolas]

It would also take a lock of the entire table and block reads

[neekolas]

I'm having problems actually running concurrent index drops in a migration (for some reason it is trying to run the migration in a tx, which is not allowed). I might just make this non-concurrent but manually drop the index concurrently in prod first.

[neekolas]

I've manually dropped message_pubsubTopic_idx, message_recvts_shouldexpire_idx, and message_sort_idx concurrently in prod. Will do the same in dev and then update this PR to include all the drops.