Create partial indexes for pending/running builds

What does this MR do?

This creates partial indexes on ci_builds for pending and running builds.

1. Issue: #327142 (closed)
2. Incident: gitlab-com/gl-infra/production#4473 (closed)

Query:

SELECT
  "ci_builds"."id"
FROM
  "ci_builds"
  INNER JOIN "projects" ON "projects"."id" = "ci_builds"."project_id"
  LEFT JOIN project_features ON ci_builds.project_id = project_features.project_id
  LEFT JOIN (
    SELECT
      "ci_builds"."project_id",
      count(*) AS running_builds
    FROM
      "ci_builds"
    WHERE
      "ci_builds"."type" = 'Ci::Build'
      AND ("ci_builds"."status" IN ('running'))
      AND "ci_builds"."runner_id" IN (
        SELECT
          "ci_runners"."id"
        FROM
          "ci_runners"
        WHERE
          "ci_runners"."runner_type" = 1)
      GROUP BY
        "ci_builds"."project_id") AS project_builds ON ci_builds.project_id = project_builds.project_id
WHERE ("ci_builds"."status" IN ('pending'))
AND "ci_builds"."runner_id" IS NULL
AND "projects"."shared_runners_enabled" = TRUE
AND "projects"."pending_delete" = FALSE
AND (project_features.builds_access_level IS NULL
  OR project_features.builds_access_level > 0)
AND "ci_builds"."type" = 'Ci::Build'
AND ("projects"."visibility_level" = 20
  OR (EXISTS ( WITH RECURSIVE "base_and_ancestors" AS ((
          SELECT
            "namespaces".*
          FROM
            "namespaces"
          WHERE (namespaces.id = projects.namespace_id))
      UNION (
        SELECT
          "namespaces".*
        FROM
          "namespaces",
          "base_and_ancestors"
        WHERE
          "namespaces"."id" = "base_and_ancestors"."parent_id"))
      SELECT DISTINCT
        1
      FROM
        "base_and_ancestors" AS "namespaces"
      LEFT JOIN namespace_statistics ON namespace_statistics.namespace_id = namespaces.id
    WHERE
      "namespaces"."parent_id" IS NULL
      AND (COALESCE(namespaces.shared_runners_minutes_limit, 400, 0) = 0
        OR COALESCE(namespace_statistics.shared_runners_seconds, 0) < COALESCE((namespaces.shared_runners_minutes_limit + COALESCE(namespaces.extra_shared_runners_minutes_limit, 0)), (400 + COALESCE(namespaces.extra_shared_runners_minutes_limit, 0)), 0) * 60))))
AND (NOT EXISTS (
    SELECT
      1
    FROM
      "taggings"
    WHERE
      "taggings"."taggable_type" = 'CommitStatus'
      AND "taggings"."context" = 'tags'
      AND (taggable_id = ci_builds.id)
      AND "taggings"."tag_id" != 26114))
AND (EXISTS (
    SELECT
      1
    FROM
      "taggings"
    WHERE
      "taggings"."taggable_type" = 'CommitStatus'
      AND "taggings"."context" = 'tags'
      AND (taggable_id = ci_builds.id)))
ORDER BY
  COALESCE(project_builds.running_builds, 0) ASC,
ci_builds.id ASC;

Query plans

1. Plan (under normal operations): https://explain.depesz.com/s/5Rjp
2. Plan (when things are on 🔥 - cause still unknown ): https://explain.depesz.com/s/QHTx
3. Plan (from dblab with indexes foobar2, foobar3 created): https://explain.depesz.com/s/2q2g

The full execution plan from dblab is here: https://explain.depesz.com/s/1e3A

After seeing this, we included the type = 'Ci::Build' into the partial condition.

Stats

1. Index sizes: Both < 11 MB
2. Creation times: Both around 9h on dblab (will be much faster in prod)

Does this MR meet the acceptance criteria?

Conformity

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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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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