Add compound index for `vulnerabilities` table on `project_id` and `id`

What does this MR do and why?

Related to #350915 (closed), and !78960 (merged).

Database review

This MR introduces a new compound index for the vulnerabilities table on project_id and id columns to speed up the SQL queries generated by the EachBatch module.

rake db:migrate:up

== 20220125230538 AddCompoundIndexOnProjectIdAndIdForVulnerabilities: migrating
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:vulnerabilities, [:project_id, :id], {:name=>"index_vulnerabilities_on_project_id_and_id", :algorithm=>:concurrently})
   -> 0.0057s
-- add_index(:vulnerabilities, [:project_id, :id], {:name=>"index_vulnerabilities_on_project_id_and_id", :algorithm=>:concurrently})
   -> 0.0047s
== 20220125230538 AddCompoundIndexOnProjectIdAndIdForVulnerabilities: migrated (0.0116s)

rake db:migrate:down

== 20220125230538 AddCompoundIndexOnProjectIdAndIdForVulnerabilities: reverting
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:vulnerabilities, [:project_id, :id], {:name=>"index_vulnerabilities_on_project_id_and_id", :algorithm=>:concurrently})
   -> 0.0064s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:vulnerabilities, {:name=>"index_vulnerabilities_on_project_id_and_id", :algorithm=>:concurrently, :column=>[:project_id, :id]})
   -> 0.0080s
-- execute("RESET statement_timeout")
   -> 0.0005s
== 20220125230538 AddCompoundIndexOnProjectIdAndIdForVulnerabilities: reverted (0.0188s)

Example queries used by the EachBatch module

Get the first vulnerability ID

SELECT
    "vulnerabilities"."id"
FROM
    "vulnerabilities"
WHERE
    "vulnerabilities"."project_id" = 278964
ORDER BY
    "vulnerabilities"."id" ASC
LIMIT 1

Before the index;

https://gitlab.slack.com/archives/CLJMDRD8C/p1643153060294400

Limit  (cost=0.44..29.35 rows=1 width=8) (actual time=4.948..4.950 rows=1 loops=1)
   Buffers: shared read=4
   I/O Timings: read=4.866 write=0.000
   ->  Index Scan using vulnerabilities_pkey on public.vulnerabilities  (cost=0.44..2381387.85 rows=82370 width=8) (actual time=4.945..4.946 rows=1 loops=1)
         Filter: (vulnerabilities.project_id = 278964)
         Rows Removed by Filter: 0
         Buffers: shared read=4
         I/O Timings: read=4.866 write=0.000

After the index;

https://gitlab.slack.com/archives/CLJMDRD8C/p1643153457300500

Limit  (cost=0.56..0.67 rows=1 width=8) (actual time=0.188..0.189 rows=1 loops=1)
   Buffers: shared hit=4 read=4
   I/O Timings: read=0.071 write=0.000
   ->  Index Only Scan using index_vulnerabilities_on_project_id_and_id on public.vulnerabilities  (cost=0.56..8741.89 rows=82009 width=8) (actual time=0.187..0.187 rows=1 loops=1)
         Index Cond: (vulnerabilities.project_id = 278964)
         Heap Fetches: 0
         Buffers: shared hit=4 read=4
         I/O Timings: read=0.071 write=0.000

Get the IDs of the next 1000 vulnerabilities

SELECT
    "vulnerabilities"."id"
FROM
    "vulnerabilities"
WHERE
    "vulnerabilities"."project_id" = 278964
    AND "vulnerabilities"."id" >= 1
ORDER BY
    "vulnerabilities"."id" ASC
LIMIT 1 OFFSET 1000

Before the index;

https://gitlab.slack.com/archives/CLJMDRD8C/p1643153157296900

Limit  (cost=28288.53..28315.81 rows=1 width=8) (actual time=104755.558..105109.059 rows=1 loops=1)
   Buffers: shared hit=1729611 read=379726 dirtied=15139 written=385
   I/O Timings: read=259374.607 write=18.595
   ->  Gather Merge  (cost=1000.46..2248718.36 rows=82370 width=8) (actual time=25.852..105108.855 rows=1001 loops=1)
         Workers Planned: 2
         Workers Launched: 2
         Buffers: shared hit=1729611 read=379726 dirtied=15139 written=385
         I/O Timings: read=259374.607 write=18.595
         ->  Parallel Index Scan using vulnerabilities_pkey on public.vulnerabilities  (cost=0.44..2238210.80 rows=34321 width=8) (actual time=7.356..88541.727 rows=336 loops=3)
               Index Cond: (vulnerabilities.id >= 1)
               Filter: (vulnerabilities.project_id = 278964)
               Rows Removed by Filter: 980598
               Buffers: shared hit=1729611 read=379726 dirtied=15139 written=385
               I/O Timings: read=259374.607 write=18.595

After the index;

https://gitlab.slack.com/archives/CLJMDRD8C/p1643153483303000

Limit  (cost=109.65..109.76 rows=1 width=8) (actual time=0.787..0.788 rows=1 loops=1)
   Buffers: shared hit=205 read=8
   I/O Timings: read=0.188 write=0.000
   ->  Index Only Scan using index_vulnerabilities_on_project_id_and_id on public.vulnerabilities  (cost=0.56..8946.91 rows=82009 width=8) (actual time=0.063..0.732 rows=1001 loops=1)
         Index Cond: ((vulnerabilities.project_id = 278964) AND (vulnerabilities.id >= 1))
         Heap Fetches: 32
         Buffers: shared hit=205 read=8
         I/O Timings: read=0.188 write=0.000

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.