Delete tracking records on partitioning rollback

What does this MR do?

Related issue: #218428 (closed)

MR !35913 (merged) added tracking for background migrations in the background_migration_jobs table. The partitioning migration helpers will be the first background migration to use the new code to track execution of background jobs. In the event of a rollback of a partitioning migration, the database should be restored to the state prior to the migration, but the initial implementation did not handle that cleanup.

This MR implements the missing functionality to delete the matching rows from background_migration_jobs.

Delete statement

DELETE FROM "background_migration_jobs" WHERE "background_migration_jobs"."class_name" = 'Gitlab::Database::PartitioningMigrationHelpers::BackfillPartitionedTable' AND (arguments ->> 2 = 'audit_events')

With the partial index:

Delete on background_migration_jobs  (cost=0.12..8.25 rows=2 width=6)
   ->  Index Scan using index_background_migration_jobs_for_partitioning_migrations on background_migration_jobs  (cost=0.12..8.25 rows=2 width=6)
         Index Cond: ((arguments ->> 2) = 'audit_events'::text)
         Filter: (class_name = 'Gitlab::Database::PartitioningMigrationHelpers::BackfillPartitionedTable'::text)

The index isn't strictly needed at this time for the single migration we have, but will be useful to have later. The initial audit_events migration will have ~6300 entries in the background_migration_jobs table.

Migration

A new index was added to support lookup of partitioning migration jobs, and the outputs for the execution are displayed below. The non-concurrent forms of add/remove index can safely be used since the table is empty at this time.

up

rails db:migrate:up VERSION=20200713152443
== 20200713152443 AddBackgroundMigrationJobIndexForPartitioningMigrations: migrating
-- add_index(:background_migration_jobs, "((arguments ->> 2))", {:name=>"index_background_migration_jobs_for_partitioning_migrations", :where=>"class_name = 'Gitlab::Database::PartitioningMigrationHelpers::BackfillPartitionedTable'"})
   -> 0.0246s
== 20200713152443 AddBackgroundMigrationJobIndexForPartitioningMigrations: migrated (0.0247s)

down

rails db:migrate:down VERSION=20200713152443
== 20200713152443 AddBackgroundMigrationJobIndexForPartitioningMigrations: reverting
-- remove_index(:background_migration_jobs, {:name=>"index_background_migration_jobs_for_partitioning_migrations"})
   -> 0.0012s
== 20200713152443 AddBackgroundMigrationJobIndexForPartitioningMigrations: reverted (0.0012s)

Does this MR meet the acceptance criteria?

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• Changelog entry
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