Use partial NOT NULL index for token_encrypted field

db/migrate/20181129104944_add_index_to_ci_builds_token_encrypted.rb

@@ -8,7 +8,7 @@ class AddIndexToCiBuildsTokenEncrypted < ActiveRecord::Migration[5.0]
   disable_ddl_transaction!
 
   def up
-    add_concurrent_index :ci_builds, :token_encrypted, unique: true
+    add_concurrent_index :ci_builds, :token_encrypted, unique: true, where: 'token_encrypted IS NOT NULL'
   end
 
   def down

db/schema.rb

@@ -361,7 +361,7 @@ ActiveRecord::Schema.define(version: 20181129104944) do
     t.index ["stage_id"], name: "index_ci_builds_on_stage_id", using: :btree
     t.index ["status", "type", "runner_id"], name: "index_ci_builds_on_status_and_type_and_runner_id", using: :btree
     t.index ["token"], name: "index_ci_builds_on_token", unique: true, using: :btree
-    t.index ["token_encrypted"], name: "index_ci_builds_on_token_encrypted", unique: true, using: :btree
+    t.index ["token_encrypted"], name: "index_ci_builds_on_token_encrypted", unique: true, where: "(token_encrypted IS NOT NULL)", using: :btree
     t.index ["updated_at"], name: "index_ci_builds_on_updated_at", using: :btree
     t.index ["user_id"], name: "index_ci_builds_on_user_id", using: :btree
   end