使用 class 策略時如何提高 Hibernate 多態查詢的性能？

Question

按 class 策略使用表時，多態查詢如下所示：

SELECT <some cols> 
FROM 
  (
    SELECT 
      parent_id,
      entity1_col,
      entity2_col,
      1 as clazz_ 
    FROM 
      entity_table1 
    UNION 
    SELECT 
      parent_id,
      entity1_col,
      entity2_col,
      2 as clazz_ 
    FROM 
      entity_table2 
  ) abstract_entity 
WHERE 
  abstract_entity.parent_id = X

我們可以看到，如果 entity_table1 和 entity_table2 中有很多行，則此查詢可能會導致性能問題。

我正在使用 Spring-data 和 Hibernate 並且在獲取父實體上的 OneToMany 關系時會生成此請求。

我的問題是：為什么 Hibernate 會生成這樣的請求，盡管它限制了 entity_table1 和 entity_table2 共有的 parent_id 上的選定行？

有沒有辦法讓 Hibernate 生成一個 SQL 請求，如下所示：

SELECT <some cols>
FROM 
  (
    SELECT 
      parent_id,
      entity1_col,
      entity2_col,
      1 as clazz_ 
    FROM 
      entity_table1 
    WHERE 
      parent_id=X
    UNION 
    SELECT 
      parent_id,
      entity1_col,
      entity2_col,
      2 as clazz_ 
    FROM 
      entity_table2 
    WHERE 
      parent_id=X
  ) abstract_entity 

FROM 中的每個請求中都有“WHERE parent_id=X”？ 這樣在獲取父實體上的關系時不會出現性能問題。

謝謝 ！

Answer 1

我根據 hibernate 5.6.3 進行了修改，以在使用每個 class 策略的表時下推多態查詢的預測。 請參考https://github.com/kerler/hibernate-orm/tree/main_5.6.3-open-care 。

修改后，對於代碼“Query q = s.createQuery("from Being h where h.identity =:name1 or h.identity =:name2 or h.identity =:name2" );” in hibernate UT case UnionSubclassTest.java::testNestedUnionedSubclasses(), the SQL generated by hibernate is like:

00:37:43,945 DEBUG SQL:144 - 
    select
        being0_.bid as bid1_2_,
        being0_.ident as ident2_2_,
        being0_.location as location3_2_,
        being0_.sex as sex1_6_,
        being0_.salary as salary1_4_,
        being0_.species as species1_0_,
        being0_.hive as hive2_0_,
        being0_.clazz_ as clazz_ 
    from
        ( select
            bid,
            ident,
            location,
            sex,
            salary,
            null as species,
            null as hive,
            2 as clazz_ 
        from
            employees 
        where
            ident=? 
            or ident=? 
            or ident=? 
        union
        all select
            bid,
            ident,
            location,
            sex,
            null as salary,
            null as species,
            null as hive,
            1 as clazz_ 
        from
            humans 
        where
            ident=? 
            or ident=? 
            or ident=? 
        union
        all select
            bid,
            ident,
            location,
            null as sex,
            null as salary,
            species,
            hive,
            3 as clazz_ 
        from
            aliens 
        where
            ident=? 
            or ident=? 
            or ident=? 
    ) being0_ 
where
    being0_.ident=? 
    or being0_.ident=? 
    or being0_.ident=?