Optimize database query for last deployment

What does this MR do?

This MR fixes the query statement timeout issue on Environment#last_deployment.

Currently, the executed query looks like this:

SELECT
    "deployments".*
FROM
    "deployments"
WHERE
    "deployments"."environment_id" = 693272
    AND "deployments"."status" = 2
ORDER BY
    deployments.id DESC
LIMIT
    1;

which produces the following query plan on db-lab:

 Limit  (cost=0.57..3.38 rows=1 width=139) (actual time=64952.563..64952.566 rows=1 loops=1)
   Buffers: shared hit=1789 read=70646 dirtied=482
   I/O Timings: read=64229.280
   ->  Index Scan using index_deployments_on_environment_id_and_id on public.deployments  (cost=0.57..109970.76 rows=39200 width=139) (actual time=64952.560..64952.561 rows=1 loops=1)
         Index Cond: (deployments.environment_id = 693272)
         Filter: (deployments.status = 2)
         Rows Removed by Filter: 72178
         Buffers: shared hit=1789 read=70646 dirtied=482
         I/O Timings: read=64229.280

It seems index_deployments_on_environment_id_status_and_finished_at is not used. Probably because it's sorted by deployments.id.

In this MR, we'll introduce a new index on (environment_id, status, id). With this index, the query plan will look like:

 Limit  (cost=0.57..1.68 rows=1 width=139) (actual time=0.664..0.665 rows=1 loops=1)
   Buffers: shared hit=4 read=4
   I/O Timings: read=0.522
   ->  Index Scan using dosuken_test on public.deployments  (cost=0.57..43722.10 rows=39304 width=139) (actual time=0.658..0.658 rows=1 loops=1)
         Index Cond: ((deployments.environment_id = 693272) AND (deployments.status = 2))
         Buffers: shared hit=4 read=4
         I/O Timings: read=0.522

It's fast enough with cold cacche.

Migration

INFO: This script is a predefined script in devkitkat.
== 20210331145548 AddIndexForLastDeployment: reverting ========================
-- transaction_open?()
   -> 0.0000s
-- indexes(:deployments)
   -> 0.0089s
-- remove_index(:deployments, {:algorithm=>:concurrently, :name=>"index_deployments_on_environment_id_status_and_id"})
   -> 0.0047s
== 20210331145548 AddIndexForLastDeployment: reverted (0.0144s) ===============

== 20210331145548 AddIndexForLastDeployment: migrating ========================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:deployments, [:environment_id, :status, :id], {:name=>"index_deployments_on_environment_id_status_and_id", :algorithm=>:concurrently})
   -> 0.0059s
-- add_index(:deployments, [:environment_id, :status, :id], {:name=>"index_deployments_on_environment_id_status_and_id", :algorithm=>:concurrently})
   -> 0.0083s
== 20210331145548 AddIndexForLastDeployment: migrated (0.0152s) ===============

Closes #325552 (closed)

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