Add unique index on job_id and file_type

db/migrate/20171130145523_uniqueness_contraint_job_artifact_file_type.rb

+class UniquenessContraintJobArtifactFileType < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :ci_job_artifacts, [:job_id, :file_type], unique: true
+  end
+
+  def down
+    remove_concurrent_index :ci_job_artifacts, [:job_id, :file_type]
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20171124150326) do
+ActiveRecord::Schema.define(version: 20171130145523) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -330,6 +330,7 @@ ActiveRecord::Schema.define(version: 20171124150326) do
   end
 
   add_index "ci_job_artifacts", ["file_type"], name: "index_ci_job_artifacts_on_file_type", using: :btree
+  add_index "ci_job_artifacts", ["job_id", "file_type"], name: "index_ci_job_artifacts_on_job_id_and_file_type", unique: true, using: :btree
   add_index "ci_job_artifacts", ["job_id"], name: "index_ci_job_artifacts_on_job_id", using: :btree
   add_index "ci_job_artifacts", ["project_id"], name: "index_ci_job_artifacts_on_project_id", using: :btree