单表和连表？如何选择？

快乐，使生命得以延续。快乐，是精神和肉体的朝气，是希望和信念，是对自己的现在和来来的信心，是一切都该如此进行的信心。——果戈理

今天做了个小测试啊

我自己造了一百万多条(1029708条)数据

这里测试呢我们首先是编写了一个LEFT JOIN

连表SQL如下

SELECT *
FROM `film`
         LEFT JOIN `language` ON `film`.language_id = `language`.language_id

我们查询一百万多条后耗时为33457.8317 ms，大约30来秒，这是没有加索引的情况下

我们使用单表查询，然后再拼装

long startTime = System.nanoTime();
List<Film> films = filmMapper.selectList(Wrappers.lambdaQuery());
List<Integer> languageIds = films.parallelStream().map(Film::getLanguageId).distinct().collect(Collectors.toList());
final Map<Integer, Language> languageMap = languageMapper.selectList(Wrappers.lambdaQuery(Language.builder().build()).in(Language::getLanguageId, languageIds)).parallelStream().collect(Collectors.toMap(Language::getLanguageId, Function.identity(), (v1, v2) -> v2));
List<Map<String, Object>> collect = films.stream().map(film -> {
    Map<String, Object> map = BeanUtils.beanToMap(film);
    Optional.ofNullable(film).map(Film::getLanguageId).map(languageMap::get).map(BeanUtils::beanToMap).ifPresent(map::putAll);
    return map;
}).collect(Collectors.toList());
long endTime = System.nanoTime();
System.out.println("耗时：" + ((endTime - startTime) / (1000.0 * 1000.0)) + " ms"); 

最后耗时为22289.5385 ms，大约20来秒，可以看到明显比上面的连表查询要快

那么如果再连一次呢，模拟两个LEFT JOIN的场景

SELECT *
FROM `film`
         LEFT JOIN `language` ON `film`.language_id = `language`.language_id
         LEFT JOIN `language` AS language1 ON `film`.language_id = language1.language_id

这里耗时37053.9295 ms，因为我们language表数据量较小，所以再连一次差别也并不是特别大

但可以明显看出，多了4秒左右

我们写成单表的话

long startTime = System.nanoTime();
     List<Film> films = filmMapper.selectList(Wrappers.lambdaQuery());
     List<Integer> languageIds = films.parallelStream().map(Film::getLanguageId).distinct().collect(Collectors.toList());
     CompletableFuture<Map<Integer, Language>> languageMapFuture = CompletableFuture.supplyAsync(() -> languageMapper.selectList(Wrappers.lambdaQuery(Language.builder().build()).in(Language::getLanguageId, languageIds)).parallelStream().collect(Collectors.toMap(Language::getLanguageId, Function.identity(), (v1, v2) -> v2)));
     CompletableFuture<Map<Integer, Language>> languageMapFuture1 = CompletableFuture.supplyAsync(() -> languageMapper.selectList(Wrappers.lambdaQuery(Language.builder().build()).in(Language::getLanguageId, languageIds)).parallelStream().collect(Collectors.toMap(Language::getLanguageId, Function.identity(), (v1, v2) -> v2)));
     CompletableFuture.allOf(languageMapFuture, languageMapFuture1).get();
     Map<Integer, Language> languageMap = languageMapFuture.get();
     Map<Integer, Language> languageMap1 = languageMapFuture1.get();
     List<Map<String, Object>> collect = films.stream().map(film -> {
         Map<String, Object> map = BeanUtils.beanToMap(film);
         Optional.ofNullable(film).map(Film::getLanguageId).map(languageMap::get).map(BeanUtils::beanToMap).ifPresent(map::putAll);
         Optional.ofNullable(film).map(Film::getLanguageId).map(languageMap1::get).map(BeanUtils::beanToMap).ifPresent(map::putAll);
         return map;
     }).collect(Collectors.toList());
     long endTime = System.nanoTime();
     System.out.println("耗时：" + ((endTime - startTime) / (1000.0 * 1000.0)) + " ms"); 

执行后耗时23362.9739 ms！！发现仅仅多了一秒左右啊

上面的连表SQL，就算在language表的language_id上加了索引，也是耗时35314.184 ms

也远远没有我们的单表快

所以结论：

同样的数据，单表多次查询在正确使用下，比连表确实快不少

但连表只需要一条SQL而单表需要写一大堆代码/******************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************