Add index on file_store for pages_deployments table

What does this MR do?

This MR adds an index on the file_store column for the pages_deployments table. This needs was discussed as part of the code review for a rake task, which will be executing the following query:

PagesDeployment.where(file_store: ...).find_each(batch_size: batch_size)

Migration Up/Down

etienne@dell ~/src/gdk/gitlab(325285-tmp-index-to-file-store ✗) rails db:migrate:up VERSION=20210401134157
== 20210401134157 AddIndexToPagesDeployments: migrating =======================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:pages_deployments, [:file_store, :id], {:name=>"index_pages_deployments_on_file_store_and_id", :algorithm=>:concurrently})
   -> 0.0016s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- add_index(:pages_deployments, [:file_store, :id], {:name=>"index_pages_deployments_on_file_store_and_id", :algorithm=>:concurrently})
   -> 0.0308s
-- execute("RESET ALL")
   -> 0.0005s
== 20210401134157 AddIndexToPagesDeployments: migrated (0.0341s) ==============

etienne@dell ~/src/gdk/gitlab(325285-tmp-index-to-file-store ✗) rails db:migrate:down VERSION=20210401134157
== 20210401134157 AddIndexToPagesDeployments: reverting =======================
-- transaction_open?()
   -> 0.0000s
-- indexes(:pages_deployments)
   -> 0.0020s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:pages_deployments, {:algorithm=>:concurrently, :name=>"index_pages_deployments_on_file_store_and_id"})
   -> 0.0343s
-- execute("RESET ALL")
   -> 0.0007s
== 20210401134157 AddIndexToPagesDeployments: reverted (0.0383s) ==============

Explain plans

Before index creation

EXPLAIN SELECT id FROM pages_deployments WHERE pages_deployments.file_store = 2 
AND id >= 12 AND id <= 47491; # 1000 pages_deployments records in prod

Index Scan using pages_deployments_pkey on public.pages_deployments  (cost=0.42..656.97 rows=1001 width=8) (actual time=0.031..129.598 rows=1001 loops=1)
   Index Cond: ((pages_deployments.id >= 12) AND (pages_deployments.id <= 47491))
   Filter: (pages_deployments.file_store = 2)
   Rows Removed by Filter: 0
   Buffers: shared hit=188 read=117
   I/O Timings: read=126.588

Time: 130.399 ms
  - planning: 0.658 ms
  - execution: 129.741 ms
    - I/O read: 126.588 ms
    - I/O write: N/A

Shared buffers:
  - hits: 188 (~1.50 MiB) from the buffer pool
  - reads: 117 (~936.00 KiB) from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

After index creation

EXEC CREATE INDEX index_pages_deployments_on_file_store_and_id
ON pages_deployments USING btree (file_store, id);

EXPLAIN SELECT id FROM pages_deployments
WHERE pages_deployments.file_store = 2 AND id >= 12 and id <= 47491;

Index Only Scan using index_pages_deployments_on_file_store_and_id on public.pages_deployments  (cost=0.42..62.23 rows=1001 width=8) (actual time=0.224..0.638 rows=1001 loops=1)
   Index Cond: ((pages_deployments.file_store = 2) AND (pages_deployments.id >= 12) AND (pages_deployments.id <= 47491))
   Heap Fetches: 52
   Buffers: shared hit=40 read=6
   I/O Timings: read=0.208

Time: 1.571 ms
  - planning: 0.865 ms
  - execution: 0.706 ms
    - I/O read: 0.208 ms
    - I/O write: N/A

Shared buffers:
  - hits: 40 (~320.00 KiB) from the buffer pool
  - reads: 6 (~48.00 KiB) from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #325285 (closed)