Remove an unneeded index on ci_builds left in some databases

Migration 20170919211300_remove_temporary_ci_builds_index.rb created a
temporary partial index and tried to drop it at the end of the
migration. In some circumstances apparently it failed to drop the
index and it ended up in our schema.rb.

This accidentally failed to fail due to a bug in the regular
expression for partial indexes which caused the index creation in
schema.rb to be ignored. Now that that's fixed we could be
resurrecting this zombie index from the past in some but not all
databases.

Add a migration to drop this index if it's present to reconcile this
discrepancy.

changelogs/unreleased/remove-temporary-ci-index.yml

+---
+title: Remove an index on ci_builds meant to be only temporary
+merge_request:
+author:
+type: other

db/migrate/20170919211300_remove_temporary_ci_builds_index.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class RemoveTemporaryCiBuildsIndex < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  # To use create/remove index concurrently
+  disable_ddl_transaction!
+
+  def up
+    return unless index_exists?(:ci_builds, :id, name: 'index_for_ci_builds_retried_migration')
+    remove_concurrent_index(:ci_builds, :id, name: "index_for_ci_builds_retried_migration")
+  end
+
+  def down
+    # this was a temporary index for a migration that was never
+    # present previously so this probably shouldn't be here but it's
+    # easier to test the drop if we have a way to create it.
+    add_concurrent_index("ci_builds", ["id"],
+                         name: "index_for_ci_builds_retried_migration",
+                         where: "(retried IS NULL)",
+                         using: :btree)
+  end
+end

db/schema.rb

@@ -256,7 +256,6 @@ ActiveRecord::Schema.define(version: 20170921115009) do
   add_index "ci_builds", ["commit_id", "status", "type"], name: "index_ci_builds_on_commit_id_and_status_and_type", using: :btree
   add_index "ci_builds", ["commit_id", "type", "name", "ref"], name: "index_ci_builds_on_commit_id_and_type_and_name_and_ref", using: :btree
   add_index "ci_builds", ["commit_id", "type", "ref"], name: "index_ci_builds_on_commit_id_and_type_and_ref", using: :btree
-  add_index "ci_builds", ["id"], name: "index_for_ci_builds_retried_migration", where: "(retried IS NULL)", using: :btree
   add_index "ci_builds", ["project_id"], name: "index_ci_builds_on_project_id", using: :btree
   add_index "ci_builds", ["protected"], name: "index_ci_builds_on_protected", using: :btree
   add_index "ci_builds", ["runner_id"], name: "index_ci_builds_on_runner_id", using: :btree