MM-45956: Optimize FileInfo stats query (#22603)

* MM-45956: Optimize FileInfo stats query

We Denormalize Post.ChannelId on FileInfo.ChannelId

```release-note
The file info stats query is now optimized by denormalizing the channelID column into the table itself. This will speed up the query to get the file count for a channel on clicking the RHS.

Migration times:
On a MySQL 8.0.31 DB with
1405 rows in FileInfo and 11M posts, it took around 0.3s

On a Postgres 12.14 DB with
1731 rows in FileInfo and 11M posts, it took around 0.27s
```

https://mattermost.atlassian.net/browse/MM-45956

server/channels/db/migrations/migrations.list

@@ -210,6 +210,8 @@ channels/db/migrations/mysql/000104_upgrade_notifyadmin.down.sql
 channels/db/migrations/mysql/000104_upgrade_notifyadmin.up.sql
 channels/db/migrations/mysql/000105_remove_tokens.down.sql
 channels/db/migrations/mysql/000105_remove_tokens.up.sql
+channels/db/migrations/mysql/000106_fileinfo_channelid.down.sql
+channels/db/migrations/mysql/000106_fileinfo_channelid.up.sql
 channels/db/migrations/postgres/000001_create_teams.down.sql
 channels/db/migrations/postgres/000001_create_teams.up.sql
 channels/db/migrations/postgres/000002_create_team_members.down.sql
@@ -420,3 +422,5 @@ channels/db/migrations/postgres/000104_upgrade_notifyadmin.down.sql
 channels/db/migrations/postgres/000104_upgrade_notifyadmin.up.sql
 channels/db/migrations/postgres/000105_remove_tokens.down.sql
 channels/db/migrations/postgres/000105_remove_tokens.up.sql
+channels/db/migrations/postgres/000106_fileinfo_channelid.down.sql
+channels/db/migrations/postgres/000106_fileinfo_channelid.up.sql

server/channels/db/migrations/mysql/000106_fileinfo_channelid.down.sql

@@ -0,0 +1,29 @@
+SET @preparedStatement = (SELECT IF(
+    EXISTS(
+        SELECT 1 FROM INFORMATION_SCHEMA.STATISTICS
+        WHERE table_name = 'FileInfo'
+        AND table_schema = DATABASE()
+        AND index_name = 'idx_fileinfo_channel_id_create_at'
+    ) > 0,
+    'DROP INDEX idx_fileinfo_channel_id_create_at ON FileInfo;',
+    'SELECT 1'
+));
+
+PREPARE removeIndexIfExists FROM @preparedStatement;
+EXECUTE removeIndexIfExists;
+DEALLOCATE PREPARE removeIndexIfExists;
+
+SET @preparedStatement = (SELECT IF(
+    EXISTS(
+        SELECT 1 FROM INFORMATION_SCHEMA.STATISTICS
+        WHERE table_name = 'FileInfo'
+        AND table_schema = DATABASE()
+        AND column_name = 'ChannelId'
+    ) > 0,
+    'ALTER TABLE FileInfo DROP COLUMN ChannelId;',
+    'SELECT 1;'
+));
+
+PREPARE removeColumnIfExists FROM @preparedStatement;
+EXECUTE removeColumnIfExists;
+DEALLOCATE PREPARE removeColumnIfExists;

server/channels/db/migrations/mysql/000106_fileinfo_channelid.up.sql

@@ -0,0 +1,34 @@
+SET @preparedStatement = (SELECT IF(
+    NOT EXISTS(
+        SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS
+        WHERE table_name = 'FileInfo'
+        AND table_schema = DATABASE()
+        AND column_name = 'ChannelId'
+    ),
+    'ALTER TABLE FileInfo ADD COLUMN ChannelId varchar(26);',
+    'SELECT 1;'
+));
+
+PREPARE addColumnIfNotExists FROM @preparedStatement;
+EXECUTE addColumnIfNotExists;
+DEALLOCATE PREPARE addColumnIfNotExists;
+
+UPDATE FileInfo, Posts
+SET FileInfo.ChannelId = Posts.ChannelId
+WHERE FileInfo.PostId = Posts.Id
+AND FileInfo.ChannelId IS NULL;
+
+SET @preparedStatement = (SELECT IF(
+    NOT EXISTS(
+        SELECT 1 FROM INFORMATION_SCHEMA.STATISTICS
+        WHERE table_name = 'FileInfo'
+        AND table_schema = DATABASE()
+        AND index_name = 'idx_fileinfo_channel_id_create_at'
+    ),
+    'CREATE INDEX idx_fileinfo_channel_id_create_at ON FileInfo(ChannelId, CreateAt);',
+    'SELECT 1'
+));
+
+PREPARE createIndexIfNotExists FROM @preparedStatement;
+EXECUTE createIndexIfNotExists;
+DEALLOCATE PREPARE createIndexIfNotExists;

server/channels/db/migrations/postgres/000106_fileinfo_channelid.down.sql

@@ -0,0 +1,2 @@
+DROP INDEX IF EXISTS idx_fileinfo_channel_id_create_at;
+ALTER TABLE fileinfo DROP COLUMN IF EXISTS channelid;

server/channels/db/migrations/postgres/000106_fileinfo_channelid.up.sql

@@ -0,0 +1,3 @@
+ALTER TABLE fileinfo ADD COLUMN IF NOT EXISTS channelid varchar(26);
+UPDATE fileinfo SET channelid = posts.channelid FROM posts WHERE fileinfo.channelid IS NULL AND fileinfo.postid = posts.id;
+CREATE INDEX IF NOT EXISTS idx_fileinfo_channel_id_create_at ON fileinfo(channelid, createat);