Mattermost 6.7: Important upgrade notes

@twk3 Below is a note of important upgrade notes in our upcoming release.

Mattermost v6.7 (GitLab 15.1 - June 22)

• New schema changes were introduced in the form of a new index. The following notes the test results for the schema changes:
  • MySQL 7M Posts - ~17s (Instance: db.r5.xlarge)
  • MySQL 9M Posts - 2min 12s (Instance: db.r5.large)
  • Postgres 7M Posts - ~9s (Instance: db.r5.xlarge)

• For customers wanting a zero downtime upgrade, they are welcome to apply this index prior to doing the upgrade. This is fully backwards compatible and will not acquire any table lock or affect any existing operations on the table.
  • For MySQL: CREATE INDEX idx_posts_create_at_id on Posts(CreateAt, Id) LOCK=NONE;
  • For Postgres: CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_posts_create_at_id on posts(createat, id);