Fix slow query of federated timeline

[notozeki]

We had found that the federated timeline was slow after upgrading our server from v2.7.3 to v3.0.1. We found an almost the same issue (#11643) and its solution (#11648), but it fixes only the local timeline. This PR fixes also the federated timeline by creating a new index similar with #11648.

EXPLAIN results of our DB:

Query

SELECT  "statuses"."id", "statuses"."updated_at" FROM "statuses" LEFT OUTER JOIN "accounts" ON "accounts"."id" = "statuses"."account_id" WHERE "statuses"."visibility" = 0 AND (statuses.reblog_of_id IS NULL) AND (statuses.reply = FALSE OR statuses.in_reply_to_account_id = statuses.account_id) AND "statuses"."account_id" NOT IN (1) AND "statuses"."deleted_at" IS NULL AND "accounts"."silenced_at" IS NULL ORDER BY "statuses"."id" DESC, "statuses"."id" DESC LIMIT 10

Before

Limit  (cost=0.72..100659.04 rows=10 width=16)
  ->  Nested Loop Left Join  (cost=0.72..1107242.22 rows=110 width=16)
        Filter: (accounts.silenced_at IS NULL)
        ->  Index Scan Backward using statuses_pkey on statuses  (cost=0.43..1051835.02 rows=22060 width=24)
              Filter: ((reblog_of_id IS NULL) AND (deleted_at IS NULL) AND ((NOT reply) OR (in_reply_to_account_id = account_id)) AND (account_id <> 1) AND (visibility = 0))
        ->  Index Scan using index_accounts_on_id on accounts  (cost=0.29..2.50 rows=1 width=16)
              Index Cond: (id = statuses.account_id)

After

Limit  (cost=0.72..9689.39 rows=10 width=16)
  ->  Nested Loop Left Join  (cost=0.72..106575.99 rows=110 width=16)
        Filter: (accounts.silenced_at IS NULL)
        ->  Index Scan using index_statuses_public_20200117 on statuses  (cost=0.43..51134.55 rows=22067 width=24)
              Filter: (account_id <> 1)
        ->  Index Scan using index_accounts_on_id on accounts  (cost=0.29..2.50 rows=1 width=16)
              Index Cond: (id = statuses.account_id)

CPU usage of our DB:

We created the index at 17:00 and CPU usage decreased immediately.

I am not sure this is the best way to fix it. Please give me a review. Thank you!