Follow-up from "Add cron worker for cleaning up unit test tables"

The following discussion from !61463 (merged) should be addressed:

This query has a nested loop inside (NOT EXISTS) which might time out if we have a lot's of ci_unit_test_failures records.

The linked plan shows that we scanned 5,238 rows to get 100 deletable UnitTest records back: https://explain-depesz.postgres.ai/s/bv

The query execution time will grow as we scan more records in the DB. We might need to alter the strategy later. Options:

• Add has_test_failure row to unit_tests (de-normalization), in order to make the scan work on a single table.
• Iterating over the table (EachBatch) and use NOT EXISTS within the block. We call this the slow iteration approach: https://docs.gitlab.com/ee/development/iterating_tables_in_batches.html#slow-iteration

Look into adding the has_test_failure column.

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mentioned in issue gitlab-org/quality/triage-reports#3131 (closed)

mentioned in issue gitlab-com/gl-infra/scalability#1068

mentioned in issue gitlab-org/quality/triage-reports#3222 (closed)

mentioned in issue gitlab-org/quality/triage-reports#3294 (closed)

mentioned in issue gitlab-org/quality/triage-reports#3385 (closed)

mentioned in issue gitlab-org/quality/triage-reports#3486 (closed)

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