refactor(vacuum statuses): reduce amount of db queries and load for each query - improve performance (#21487)

* refactor(statuses_vacuum): remove dead code - unused

Method is not called inside class and private.
Clean up dead code.

* refactor(statuses_vacuum): make retention_period present test explicit

This private method only hides functionality.
It is best practice to be as explicit as possible.

* refactor(statuses_vacuum): improve query performance

- fix statuses_scope having sub-select for Account.remote scope by
  `joins(:account).merge(Account.remote)`
- fix statuses_scope unnecessary use of `Status.arel_table[:id].lt`
  because it is inexplicit, bad practice and even slower than normal
  `.where('statuses.id < ?'`
- fix statuses_scope remove select(:id, :visibility) for having reusable
  active record query batches (no re queries)
- fix vacuum_statuses! to use in_batches instead of find_in_batches,
  because in_batches delivers a full blown active record query result,
  in stead of an array - no requeries necessary
- send(:unlink_from_conversations) not to perform another db query, but
  reuse the in_batches result instead.
- remove now obsolete remove_from_account_conversations method
- remove_from_search_index uses array of ids, instead of mapping
  the ids from an array - this should be more efficient
- use the in_batches scope to call delete_all, instead of running
  another db query for this - because it is again more efficient
- add TODO comment for calling models private method with send

* refactor(status): simplify unlink_from_conversations

- add `has_many through:` relation mentioned_accounts
- use model scope local instead of method call `Status#local?`
- more readable add account to inbox_owners when account.local?

* refactor(status): searchable_by way less sub selects

These queries all included a sub-select. Doing the same with a joins
should be more efficient.
Since this method does 5 such queries, this should be significant,
since it technically halves the query count.

This is how it was:

```ruby
[3] pry(main)> Status.first.mentions.where(account: Account.local, silent: false).explain
  Status Load (1.6ms)  SELECT "statuses".* FROM "statuses" WHERE "statuses"."deleted_at" IS NULL ORDER BY "statuses"."id" DESC LIMIT $1  [["LIMIT", 1]]
  Mention Load (1.5ms)  SELECT "mentions".* FROM "mentions" WHERE "mentions"."status_id" = $1 AND "mentions"."account_id" IN (SELECT "accounts"."id" FROM "accounts" WHERE "accounts"."domain" IS NULL) AND "mentions"."silent" = $2  [["status_id", 109382923142288414], ["silent", false]]
=> EXPLAIN for: SELECT "mentions".* FROM "mentions" WHERE "mentions"."status_id" = $1 AND "mentions"."account_id" IN (SELECT "accounts"."id" FROM "accounts" WHERE "accounts"."domain" IS NULL) AND "mentions"."silent" = $2 [["status_id", 109382923142288414], ["silent", false]]
                                                    QUERY PLAN
------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.15..23.08 rows=1 width=41)
   ->  Seq Scan on accounts  (cost=0.00..10.90 rows=1 width=8)
         Filter: (domain IS NULL)
   ->  Index Scan using index_mentions_on_account_id_and_status_id on mentions  (cost=0.15..8.17 rows=1 width=41)
         Index Cond: ((account_id = accounts.id) AND (status_id = '109382923142288414'::bigint))
         Filter: (NOT silent)
(6 rows)
```

This is how it is with this change:

```ruby
[4] pry(main)> Status.first.mentions.joins(:account).merge(Account.local).active.explain
  Status Load (1.7ms)  SELECT "statuses".* FROM "statuses" WHERE "statuses"."deleted_at" IS NULL ORDER BY "statuses"."id" DESC LIMIT $1  [["LIMIT", 1]]
  Mention Load (0.7ms)  SELECT "mentions".* FROM "mentions" INNER JOIN "accounts" ON "accounts"."id" = "mentions"."account_id" WHERE "mentions"."status_id" = $1 AND "accounts"."domain" IS NULL AND "mentions"."silent" = $2  [["status_id", 109382923142288414], ["silent", false]]
=> EXPLAIN for: SELECT "mentions".* FROM "mentions" INNER JOIN "accounts" ON "accounts"."id" = "mentions"."account_id" WHERE "mentions"."status_id" = $1 AND "accounts"."domain" IS NULL AND "mentions"."silent" = $2 [["status_id", 109382923142288414], ["silent", false]]
                                                    QUERY PLAN
------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.15..23.08 rows=1 width=41)
   ->  Seq Scan on accounts  (cost=0.00..10.90 rows=1 width=8)
         Filter: (domain IS NULL)
   ->  Index Scan using index_mentions_on_account_id_and_status_id on mentions  (cost=0.15..8.17 rows=1 width=41)
         Index Cond: ((account_id = accounts.id) AND (status_id = '109382923142288414'::bigint))
         Filter: (NOT silent)
(6 rows)
```

1 files changed, 8 insertions, 7 deletions

diff --git a/app/models/status.rb b/app/models/status.rb
index 8bdb5e8db..2fe9f2de0 100644
--- a/app/models/status.rb
+++ b/app/models/status.rb
@@ -66,6 +66,7 @@ class Status < ApplicationRecord
   has_many :reblogged_by_accounts, through: :reblogs, class_name: 'Account', source: :account
   has_many :replies, foreign_key: 'in_reply_to_id', class_name: 'Status', inverse_of: :thread
   has_many :mentions, dependent: :destroy, inverse_of: :status
+  has_many :mentioned_accounts, through: :mentions, source: :account, class_name: 'Account'
   has_many :active_mentions, -> { active }, class_name: 'Mention', inverse_of: :status
   has_many :media_attachments, dependent: :nullify
 
@@ -141,11 +142,11 @@ class Status < ApplicationRecord
     ids << account_id if local?
 
     if preloaded.nil?
-      ids += mentions.where(account: Account.local, silent: false).pluck(:account_id)
-      ids += favourites.where(account: Account.local).pluck(:account_id)
-      ids += reblogs.where(account: Account.local).pluck(:account_id)
-      ids += bookmarks.where(account: Account.local).pluck(:account_id)
-      ids += poll.votes.where(account: Account.local).pluck(:account_id) if poll.present?
+      ids += mentions.joins(:account).merge(Account.local).active.pluck(:account_id)
+      ids += favourites.joins(:account).merge(Account.local).pluck(:account_id)
+      ids += reblogs.joins(:account).merge(Account.local).pluck(:account_id)
+      ids += bookmarks.joins(:account).merge(Account.local).pluck(:account_id)
+      ids += poll.votes.joins(:account).merge(Account.local).pluck(:account_id) if poll.present?
     else
       ids += preloaded.mentions[id] || []
       ids += preloaded.favourites[id] || []
@@ -527,8 +528,8 @@ class Status < ApplicationRecord
   def unlink_from_conversations
     return unless direct_visibility?
 
-    mentioned_accounts = (association(:mentions).loaded? ? mentions : mentions.includes(:account)).map(&:account)
-    inbox_owners       = mentioned_accounts.select(&:local?) + (account.local? ? [account] : [])
+    inbox_owners = mentioned_accounts.local
+    inbox_owners += [account] if account.local?
 
     inbox_owners.each do |inbox_owner|
       AccountConversation.remove_status(inbox_owner, self)