Add IID to Feature Flags and Backfill with Param in AtomicInternalId

What does this MR do?

Implementation of !20871 (closed) using SQL and a backfill parameter in AtomicInternalId.

Migration Performance

Running the backfill in #database-lab yields the following results:

EXEC ALTER TABLE "operations_feature_flags" ADD "iid" integer

EXEC CREATE UNIQUE INDEX CONCURRENTLY "index_operations_feature_flags_on_project_id_and_iid" ON "operations_feature_flags" ("project_id", "iid")

EXPLAIN UPDATE operations_feature_flags SET iid = operations_feature_flags_with_calculated_iid.iid_num FROM (SELECT id, ROW_NUMBER() OVER (PARTITION BY project_id ORDER BY id ASC) AS iid_num FROM operations_feature_flags) AS operations_feature_flags_with_calculated_iid WHERE operations_feature_flags.id = operations_feature_flags_with_calculated_iid.id AND operations_feature_flags.iid IS NULL

 ModifyTable on public.operations_feature_flags  (cost=84.60..104.24 rows=3 width=112) (actual time=19.396..19.396 rows=0 loops=1)
   Buffers: shared hit=6745 read=24 dirtied=39
   I/O Timings: read=9.105
   ->  Hash Join  (cost=84.60..104.24 rows=3 width=112) (actual time=0.836..1.732 rows=581 loops=1)
         Hash Cond: (operations_feature_flags_with_calculated_iid.id = operations_feature_flags.id)
         Buffers: shared hit=297
         ->  Subquery Scan  (cost=64.48..81.91 rows=581 width=56) (actual time=0.311..0.893 rows=581 loops=1)
               Buffers: shared hit=14
               ->  WindowAgg  (cost=64.48..76.10 rows=581 width=20) (actual time=0.290..0.693 rows=581 loops=1)
                     Buffers: shared hit=14
                     ->  Sort  (cost=64.48..65.94 rows=581 width=12) (actual time=0.279..0.358 rows=581 loops=1)
                           Sort Key: operations_feature_flags_1.project_id, operations_feature_flags_1.id
                           Sort Method: quicksort  Memory: 52kB
                           Buffers: shared hit=14
                           ->  Seq Scan on public.operations_feature_flags operations_feature_flags_1  (cost=0.00..37.81 rows=581 width=12) (actual time=0.005..0.077 rows=581 loops=1)
                                 Buffers: shared hit=8
         ->  Hash  (cost=20.07..20.07 rows=3 width=68) (actual time=0.504..0.504 rows=581 loops=1)
               Buckets: 1024  Batches: 1  Memory Usage: 68kB
               Buffers: shared hit=283
               ->  Index Scan using index_operations_feature_flags_on_project_id_and_iid on public.operations_feature_flags  (cost=0.28..20.07 rows=3 width=68) (actual time=0.052..0.285 rows=581 loops=1)
                     Index Cond: (operations_feature_flags.iid IS NULL)
                     Buffers: shared hit=283

Time: 20.496 ms
  - planning: 0.976 ms
  - execution: 19.520 ms
    - I/O read: 9.105 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 6745 (~52.70 MiB) from the buffer pool
  - reads: 24 (~192.00 KiB) from the OS file cache, including disk I/O
  - dirtied: 39 (~312.00 KiB)
  - writes: 0

It is expected to backfill around 700 rows on gitlab.com:

gitlabhq_production=> SELECT COUNT(id) FROM operations_feature_flags;
 count 
-------
   676
(1 row)

gitlabhq_production=> 

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

Security

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• 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