Change partial index's condition to make stale delayed builds removal a lot faster

db/migrate/20180924201039_add_partial_index_to_scheduled_at.rb

@@ -9,7 +9,7 @@ class AddPartialIndexToScheduledAt < ActiveRecord::Migration
   disable_ddl_transaction!
 
   def up
-    add_concurrent_index(:ci_builds, [:scheduled_at, :id], where: "scheduled_at IS NOT NULL", name: INDEX_NAME)
+    add_concurrent_index(:ci_builds, :scheduled_at, where: "scheduled_at IS NOT NULL AND type = 'Ci::Build' AND status = 'scheduled'", name: INDEX_NAME)
   end
 
   def down

db/schema.rb

@@ -347,7 +347,7 @@ ActiveRecord::Schema.define(version: 20180924201039) do
   add_index "ci_builds", ["project_id", "id"], name: "index_ci_builds_on_project_id_and_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
-  add_index "ci_builds", ["scheduled_at", "id"], name: "partial_index_ci_builds_on_scheduled_at_with_scheduled_jobs", where: "(scheduled_at IS NOT NULL)", using: :btree
+  add_index "ci_builds", ["scheduled_at"], name: "partial_index_ci_builds_on_scheduled_at_with_scheduled_jobs", where: "((scheduled_at IS NOT NULL) AND ((type)::text = 'Ci::Build'::text) AND ((status)::text = 'scheduled'::text))", using: :btree
   add_index "ci_builds", ["stage_id", "stage_idx"], name: "tmp_build_stage_position_index", where: "(stage_idx IS NOT NULL)", using: :btree
   add_index "ci_builds", ["stage_id"], name: "index_ci_builds_on_stage_id", using: :btree
   add_index "ci_builds", ["status", "type", "runner_id"], name: "index_ci_builds_on_status_and_type_and_runner_id", using: :btree
@@ -2290,6 +2290,7 @@ ActiveRecord::Schema.define(version: 20180924201039) do
   add_foreign_key "boards", "namespaces", column: "group_id", on_delete: :cascade
   add_foreign_key "boards", "projects", name: "fk_f15266b5f9", on_delete: :cascade
   add_foreign_key "chat_teams", "namespaces", on_delete: :cascade
+  add_foreign_key "ci_build_schedules", "ci_builds", column: "build_id", on_delete: :cascade
   add_foreign_key "ci_build_trace_chunks", "ci_builds", column: "build_id", on_delete: :cascade
   add_foreign_key "ci_build_trace_section_names", "projects", on_delete: :cascade
   add_foreign_key "ci_build_trace_sections", "ci_build_trace_section_names", column: "section_name_id", name: "fk_264e112c66", on_delete: :cascade