Fully shard todos

Todos (# rows: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/42475/commands/130022)

Milestone 18.5 (required stop)

• identify the source of num_nonnulls( project_id, group_id ) > 1 -> https://postgres.ai/console/gitlab/gitlab-production-main/sessions/42475/commands/130020
  • Cause: https://gitlab.com/gitlab-org/gitlab/-/blob/dfabe64d474445f57a3ac588ce4a2391b2a7f5df/app/models/todo.rb#L80-81
• identify the source of num_nonnulls( project_id, group_id ) < 1 -> https://postgres.ai/console/gitlab/gitlab-production-main/sessions/43299/commands/132277
  • Cause: https://gitlab.com/gitlab-org/gitlab/-/blob/dfabe64d474445f57a3ac588ce4a2391b2a7f5df/app/models/todo.rb#L80-81
• Add organization_id column -> !204713 (merged)
  • add column organization_id
  • prepare async index on organization_id
• verify the index on organization_id has been created on gitlab.com
• Set sharding key column -> !204559 (merged)
  • add concurrent index on organization_id
  • add concurrent foreign key constraint (NOT VALID) on organization_id
  • backfill rows that contain num_nonnulls( project_id, group_id, organization_id ) > 1
  • backfill rows that contain num_nonnulls( project_id, group_id, organization_id ) < 1
  • modify the rails model to prefer project_id over group_id when both are set
  • modify the rails model to set organization_id when project and group are nil (i.e parentless?)
  • set NOT NULL (NOT VALID) on sharding key
• verify the foreign key constraint (NOT VALID) on organization_id has been created on gitlab.com
• validate foreign key constraint (ASYNC) -> !208310 (merged)
• verify the foreign key constraint is present on the table's sharding key column
• verify sharding key backfills completed successfully
• verify 0 rows where sharding key columns are num_nonnulls( organization_id, project_id, group_id ) != 1 -> https://postgres.ai/console/gitlab/gitlab-production-main/sessions/44711/commands/137256

Milestone 18.6

• Prepare to fully shard table -> !209494 (merged)
  • validate foreign key constraint (SYNC)
  • finalize sharding key backfill
  • prepare ASYNC constraint check on NOT NULL sharding key
• Verify the NOT NULL (NOT VALID) -> NOT NULL constraint has changed on the sharding key on .com
• Fully shard table -> !211275 (merged)
  • SYNC validate NOT NULL constraint on sharding key
  • update db/docs/*.yml with sharding key
• verify the NOT NULL constraint check is set on the table's sharding key