Revert "Remove index_exists? guard"

This reverts commit d76807838cf06e023b608c16426174a752a8ccf5.

db/post_migrate/20180427161409_migrate_legacy_artifacts_to_job_artifacts.rb

@@ -33,7 +33,9 @@ class MigrateLegacyArtifactsToJobArtifacts < ActiveRecord::Migration
     # We add an temporary index to `ci_builds.artifacts_file` column to avoid statements timeout
     # This index is to be removed after we have cleaned up background migrations
     # https://gitlab.com/gitlab-org/gitlab-ce/issues/46866
-    add_concurrent_index :ci_builds, :artifacts_file, where: "artifacts_file <> ''"
+    unless index_exists?(:ci_builds, :artifacts_file)
+      add_concurrent_index :ci_builds, :artifacts_file, where: "artifacts_file <> ''"
+    end
     MigrateLegacyArtifactsToJobArtifacts::Build.legacy_artifacts.without_new_artifacts.tap do |relation|
       queue_background_migration_jobs_by_range_at_intervals(relation,
@@ -44,6 +46,8 @@ class MigrateLegacyArtifactsToJobArtifacts < ActiveRecord::Migration
   end
   def down
-    remove_concurrent_index :ci_builds, :artifacts_file, where: "artifacts_file <> ''"
+    if index_exists?(:ci_builds, :artifacts_file)
+      remove_concurrent_index :ci_builds, :artifacts_file, where: "artifacts_file <> ''"
+    end
   end
 end