为什么会多次调用DB

Question

我正在使用 Postgre SQL 来玩 R2DBC。我正在尝试的用例是通过 ID 以及语言、演员和类别获取电影。下面是架构

这是ServiceImpl中对应的一段代码

@Override
public Mono<FilmModel> getById(Long id) { 
    Mono<Film> filmMono = filmRepository.findById(id).switchIfEmpty(Mono.error(DataFormatException::new)).subscribeOn(Schedulers.boundedElastic());
    Flux<Actor> actorFlux = filmMono.flatMapMany(this::getByActorId).subscribeOn(Schedulers.boundedElastic());
    Mono<String> language = filmMono.flatMap(film -> languageRepository.findById(film.getLanguageId())).map(Language::getName).subscribeOn(Schedulers.boundedElastic());
    Mono<String> category = filmMono.flatMap(film -> filmCategoryRepository
                    .findFirstByFilmId(film.getFilmId()))
            .flatMap(filmCategory -> categoryRepository.findById(filmCategory.getCategoryId()))
            .map(Category::getName).subscribeOn(Schedulers.boundedElastic());

    return Mono.zip(filmMono, actorFlux.collectList(), language, category)
            .map(tuple -> {
                FilmModel filmModel = GenericMapper.INSTANCE.filmToFilmModel(tuple.getT1());
                List<ActorModel> actors = tuple
                        .getT2()
                        .stream()
                        .map(act -> GenericMapper.INSTANCE.actorToActorModel(act))
                        .collect(Collectors.toList());
                filmModel.setActorModelList(actors);
                filmModel.setLanguage(tuple.getT3());
                filmModel.setCategory(tuple.getT4());
                return filmModel;
            });
         }

日志显示 4 次调用拍摄

2021-12-16 21:21:20.026 DEBUG 32493 --- [ctor-tcp-nio-10] o.s.r2dbc.core.DefaultDatabaseClient     : Executing SQL statement [SELECT film.* FROM film WHERE film.film_id = $1 LIMIT 2]
2021-12-16 21:21:20.026 DEBUG 32493 --- [actor-tcp-nio-9] o.s.r2dbc.core.DefaultDatabaseClient     : Executing SQL statement [SELECT film.* FROM film WHERE film.film_id = $1 LIMIT 2]
2021-12-16 21:21:20.026 DEBUG 32493 --- [ctor-tcp-nio-12] o.s.r2dbc.core.DefaultDatabaseClient     : Executing SQL statement [SELECT film.* FROM film WHERE film.film_id = $1 LIMIT 2]
2021-12-16 21:21:20.026 DEBUG 32493 --- [actor-tcp-nio-7] o.s.r2dbc.core.DefaultDatabaseClient     : Executing SQL statement [SELECT film.* FROM film WHERE film.film_id = $1 LIMIT 2]
2021-12-16 21:21:20.162 DEBUG 32493 --- [actor-tcp-nio-9] o.s.r2dbc.core.DefaultDatabaseClient     : Executing SQL statement [SELECT language.* FROM language WHERE language.language_id = $1 LIMIT 2]
2021-12-16 21:21:20.188 DEBUG 32493 --- [actor-tcp-nio-7] o.s.r2dbc.core.DefaultDatabaseClient     : Executing SQL statement [SELECT film_actor.actor_id, film_actor.film_id, film_actor.last_update FROM film_actor WHERE film_actor.film_id = $1]
2021-12-16 21:21:20.188 DEBUG 32493 --- [ctor-tcp-nio-10] o.s.r2dbc.core.DefaultDatabaseClient     : Executing SQL statement [SELECT film_category.film_id, film_category.category_id, film_category.last_update FROM film_category WHERE film_category.film_id = $1 LIMIT 1]
2021-12-16 21:21:20.313 DEBUG 32493 --- [ctor-tcp-nio-10] o.s.r2dbc.core.DefaultDatabaseClient     : Executing SQL statement [SELECT category.* FROM category WHERE category.category_id = $1 LIMIT 2]
2021-12-16 21:21:20.563 DEBUG 32493 --- [actor-tcp-nio-7] o.s.r2dbc.core.DefaultDatabaseClient     : Executing SQL statement [SELECT actor.* FROM actor WHERE actor.actor_id = $1 LIMIT 2]

我并不是想寻找 SQL 优化（连接等）。我绝对可以使其性能更高。但问题是为什么我确实看到了 4 个对 Film 表的 SQL 查询。只是补充一下，我已经修复了代码。但无法理解核心原因。提前致谢。

Answer 1

为什么我确实看到有 4 个针对 Film 表的 SQL 查询。

原因很简单。您订阅了Mono<Film>4次：

Mono<Film> filmMono = filmRepository.findById(id);

Flux<Actor> actorFlux = filmMono.flatMapMany(...); (1)
Mono<String> language = filmMono.flatMap(...); (2)
Mono<String> category = filmMono.flatMap(...); (3)
Mono.zip(filmMono, actorFlux.collectList(), language, category) (4)

每个订阅都会filmMono触发一个新的查询。请注意，您可以通过使用Mono#cache运算符将​​其变为filmMono热源并缓存所有四个订阅者的结果来更改这一点。