Prepare Ci::JobArtifact for partitioning 1/3

Ref: #429002 (closed)

What does this MR do and why?

This MR prepares Ci::JobArtifact for partitioning and will enqueue 2 index that will be created this weekend during low traffic.

It creates 2 asynchronous indexes required to partition this table:

• Primary key: ci_jobs_artifacts (id, partition_id).
• Unique constraint ci_jobs_artifacts (job_id, file_type, partition_id).

Indexes:
    "ci_job_artifacts_pkey" PRIMARY KEY, btree (id)
    "index_ci_job_artifacts_on_job_id_and_file_type" UNIQUE, btree (job_id, file_type)

In a follow up MR we will add those indexes for self-instance users and this will create the following schema:

Indexes:
    "ci_job_artifacts_pkey" PRIMARY KEY, btree (id, partition_id)
    "idx_ci_job_artifacts_on_job_id_file_type_and_partition_id_uniq" UNIQUE, btree (job_id, file_type, partition_id)

Why are we doing this?

Mandatory steps in order to partition a table as explained in our documentation.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.