Resolve "should INDEX index_epics_on_lock_version ON epics be in init_structure.sql ?"

What does this MR do and why?

Removes the following indexes if they exist.

• index_issues_on_lock_version
• index_merge_requests_on_lock_version
• index_epics_on_lock_version

To get more context about why we are doing this. Please read this comment.

up:

== 20220106141756 RemoveIndexes: migrating ====================================
-- transaction_open?()
   -> 0.0000s
-- indexes(:issues)
   -> 0.0109s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:issues, {:algorithm=>:concurrently, :name=>"index_issues_on_lock_version"})
   -> 0.0036s
-- execute("RESET statement_timeout")
   -> 0.0005s
-- transaction_open?()
   -> 0.0000s
-- indexes(:merge_requests)
   -> 0.0094s
-- remove_index(:merge_requests, {:algorithm=>:concurrently, :name=>"index_merge_requests_on_lock_version"})
   -> 0.0023s
-- transaction_open?()
   -> 0.0000s
-- indexes(:epics)
   -> 0.0046s
-- remove_index(:epics, {:algorithm=>:concurrently, :name=>"index_epics_on_lock_version"})
   -> 0.0021s
== 20220106141756 RemoveIndexes: migrated (0.0385s) ===========================

down:

== 20220106141756 RemoveIndexes: reverting ====================================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issues, :lock_version, {:where=>"lock_version IS NULL", :name=>"index_issues_on_lock_version", :algorithm=>:concurrently})
   -> 0.0119s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:issues, :lock_version, {:where=>"lock_version IS NULL", :name=>"index_issues_on_lock_version", :algorithm=>:concurrently})
   -> 0.0027s
-- execute("RESET statement_timeout")
   -> 0.0004s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:merge_requests, :lock_version, {:where=>"lock_version IS NULL", :name=>"index_merge_requests_on_lock_version", :algorithm=>:concurrently})
   -> 0.0086s
-- add_index(:merge_requests, :lock_version, {:where=>"lock_version IS NULL", :name=>"index_merge_requests_on_lock_version", :algorithm=>:concurrently})
   -> 0.0024s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:epics, :lock_version, {:where=>"lock_version IS NULL", :name=>"index_epics_on_lock_version", :algorithm=>:concurrently})
   -> 0.0045s
-- add_index(:epics, :lock_version, {:where=>"lock_version IS NULL", :name=>"index_epics_on_lock_version", :algorithm=>:concurrently})
   -> 0.0020s
== 20220106141756 RemoveIndexes: reverted (0.0374s) ===========================

How to set up and validate locally

First part:

1. Connect to PostgreSQL / DBlab (Production)
2. Run \d+ issues, \d+ merge_requests, \d+ epics - The indexes should not be present

Second part:

1. cd gitlab-development-kit/gitlab
2. bundle exec rake db:drop db:create db:migrate
3. cd ..
4. gdk postgresql
5. Run \d+ issues, \d+ merge_requests, \d+ epics - The indexes should be present

Third part:

1. cd gitlab-development-kit/gitlab
2. git checkout 348872-should-index-index_epics_on_lock_version-on-epics-be-in-init_structure-sql
3. bundle exec rake db:drop db:create db:migrate
4. cd ..
5. gdk postgresql
6. Run \d+ issues, \d+ merge_requests, \d+ epics - The indexes should not be present

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #348872 (closed)