Add indexes and change SQL for expired artifacts to deal with artifacts migration efficiently

Artifacts are in the middle of being migrated from ci_builds to
ci_job_artifacts. The expiration date is currently visible in both of
these tables and the test for whether an expired artifact is present
for a job is complex as it requires checking both the of the tables.

Add two new indexes, one on ci_builds.artifacts_expire_at and one on
ci_job_artifacts.expire_at to enable finding expired artifacts
efficiently.

And until the migration is finished, replace the SQL for finding
expired and non-expired artifacts with a hand-crafted UNION ALL based
query instead of using OR. This overcomes a database optimizer
limitation that prevents it from using these indexes.

When the migration is finished the next version should remove this
query and replace it with a much simpler query on just
ci_job_artifacts. See
https://gitlab.com/gitlab-org/gitlab-ce/issues/42561 for followup.

1 files changed, 23 insertions, 0 deletions

diff --git a/db/migrate/20180119160751_optimize_ci_job_artifacts.rb b/db/migrate/20180119160751_optimize_ci_job_artifacts.rb
new file mode 100644
index 00000000000..9b4340ed7b7
--- /dev/null
+++ b/db/migrate/20180119160751_optimize_ci_job_artifacts.rb
@@ -0,0 +1,23 @@
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class OptimizeCiJobArtifacts < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    # job_id is just here to be a covering index for index only scans
+    # since we'll almost always be joining against ci_builds on job_id
+    add_concurrent_index(:ci_job_artifacts, [:expire_at, :job_id])
+    add_concurrent_index(:ci_builds, [:artifacts_expire_at], where: "artifacts_file <> ''")
+  end
+
+  def down
+    remove_concurrent_index(:ci_job_artifacts, [:expire_at, :job_id])
+    remove_concurrent_index(:ci_builds, [:artifacts_expire_at], where: "artifacts_file <> ''")
+  end
+end