Change to partial index

db/migrate/20181120091639_add_foreign_key_to_ci_pipelines_merge_requests.rb

@@ -8,7 +8,7 @@ class AddForeignKeyToCiPipelinesMergeRequests < ActiveRecord::Migration
   disable_ddl_transaction!
 
   def up
-    add_concurrent_index :ci_pipelines, :merge_request_id
+    add_concurrent_index :ci_pipelines, :merge_request_id, where: 'merge_request_id IS NOT NULL'
     add_concurrent_foreign_key :ci_pipelines, :merge_requests, column: :merge_request_id, on_delete: :cascade
   end
 
@@ -17,6 +17,6 @@ class AddForeignKeyToCiPipelinesMergeRequests < ActiveRecord::Migration
       remove_foreign_key :ci_pipelines, :merge_requests
     end
 
-    remove_concurrent_index :ci_pipelines, :merge_request_id
+    remove_concurrent_index :ci_pipelines, :merge_request_id, where: 'merge_request_id IS NOT NULL'
   end
 end

db/schema.rb

@@ -476,7 +476,7 @@ ActiveRecord::Schema.define(version: 20181126153547) do
     t.integer "iid"
     t.integer "merge_request_id"
     t.index ["auto_canceled_by_id"], name: "index_ci_pipelines_on_auto_canceled_by_id", using: :btree
-    t.index ["merge_request_id"], name: "index_ci_pipelines_on_merge_request_id", using: :btree
+    t.index ["merge_request_id"], name: "index_ci_pipelines_on_merge_request_id", where: "(merge_request_id IS NOT NULL)", using: :btree
     t.index ["pipeline_schedule_id"], name: "index_ci_pipelines_on_pipeline_schedule_id", using: :btree
     t.index ["project_id", "iid"], name: "index_ci_pipelines_on_project_id_and_iid", unique: true, where: "(iid IS NOT NULL)", using: :btree
     t.index ["project_id", "ref", "status", "id"], name: "index_ci_pipelines_on_project_id_and_ref_and_status_and_id", using: :btree