首页 \ 问答 \ 为什么hibernate为了加载@OneToOne双向关联而执行两个查询？(Why hibernate perform two queries for eager load a @OneToOne bidirectional association?)

为什么hibernate为了加载@OneToOne双向关联而执行两个查询？(Why hibernate perform two queries for eager load a @OneToOne bidirectional association?)

 我有实体A有一个B实体，B有一个带有@OneToOne双向关联的A。  
 现在，当我找到A记录时，hibernate用B上的左外连接执行两个查询，如下所示：  
select a.id, a.id_b, a.field1, b.id, b.field1 from A as a, B as b left outer join b ON b.id=a.id_b;
select a.id, a.id_b, a.field1, b.id, b.field1 from A as a, B as b left outer join b ON b.id=a.id_b WHERE b.id=?
 
 首先查询加载A和B字段，这是可以的，但为什么执行第二个查询来重新加载A？ 我认为这个查询加载了B中的A内容，但是这个A很明显是包含B的A ...因此它已经加载了第一个查询，是不是真的？  
  - 编辑 -   
 实体A：  
@Entity
public class A implements Serializable{
    // id and other ecc ecc
    @OneToOne
    @JoinColumn(name="id_b")
    B b;
}
 
 实体B：  
@Entity
public class B implements Serializable{
    // id and other ecc ecc
    @OneToOne(mappedBy="b")
    A a;
}
 
 情况就是这样，findAll on A需要两个查询......为什么？ 

i have entity A that has-a B entity, and B has-a A with @OneToOne bidirectional association. 
Now, when i findall A records, hibernate perform two queries with a left outer join on B, something like this: 
select a.id, a.id_b, a.field1, b.id, b.field1 from A as a, B as b left outer join b ON b.id=a.id_b;
select a.id, a.id_b, a.field1, b.id, b.field1 from A as a, B as b left outer join b ON b.id=a.id_b WHERE b.id=?
 
First query load A and B fields and it is ok, but why perform second query to reload A? I think this query load the A content in B, but this A is obviusly the A that contains B... so its already loaded with first query, isn't true? 
-- EDIT -- 
Entity A: 
@Entity
public class A implements Serializable{
    // id and other ecc ecc
    @OneToOne
    @JoinColumn(name="id_b")
    B b;
}
 
Entity B: 
@Entity
public class B implements Serializable{
    // id and other ecc ecc
    @OneToOne(mappedBy="b")
    A a;
}
 
This is the situation, and a findAll on A need two queries... why?

原文：https://stackoverflow.com/questions/3381521

更新时间：2023-07-14 17:07

最满意答案

 有两种使用TSQL选择BLOB的方法：  
SELECT * FROM OPENROWSET (BULK 'C:\Test\Test1.pdf', SINGLE_BLOB) a
 
 以及：  
SELECT BulkColumn FROM OPENROWSET (BULK 'C:\Test\Test1.pdf', SINGLE_BLOB) a
 
 请注意FROM子句之后的相关名称，这是强制性的。  
 第二个版本可以用于UPDATE，如下例所示：  
UPDATE MyTable 
SET blobField = 
   (SELECT BulkColumn FROM OPENROWSET (BULK 'C:\Test\Test1.pdf', SINGLE_BLOB) a) 
WHERE (CriteriaField = @criteria)
 
 对于部分更新，可以使用此MSDN文章中所述的SET .WRITE增变器，这里是语法：  
UPDATE MyTable SET BlobField .WRITE (expression, @offset, @length) WHERE (CriteriaField = @criteria)
 
 请注意，WRITE mutator只能用于NON-NULL字段。  
 事实上，通过将@offset设置为0并将@length设置为NULL（或实际长度），也可以将其用于执行完整更新（如果列不包含NULL），如下例所示：  
DECLARE @tmp VARBINARY(MAX) --Change to the correct datatype here
SELECT @tmp = BulkColumn FROM OPENROWSET (BULK 'C:\Test\Test1.pdf', SINGLE_BLOB) a
UPDATE MyTable SET BlobField .WRITE (@tmp, 0, NULL) WHERE (CriteriaField = @criteria)

There are two ways to SELECT a BLOB with TSQL: 
SELECT * FROM OPENROWSET (BULK 'C:\Test\Test1.pdf', SINGLE_BLOB) a
 
As well as: 
SELECT BulkColumn FROM OPENROWSET (BULK 'C:\Test\Test1.pdf', SINGLE_BLOB) a
 
Note the correlation name after the FROM clause, which is mandatory. 
The second version can be used for a UPDATE as in the following example: 
UPDATE MyTable 
SET blobField = 
   (SELECT BulkColumn FROM OPENROWSET (BULK 'C:\Test\Test1.pdf', SINGLE_BLOB) a) 
WHERE (CriteriaField = @criteria)
 
For partial updates one can use the SET .WRITE mutator as described in this MSDN article, here is the syntax: 
UPDATE MyTable SET BlobField .WRITE (expression, @offset, @length) WHERE (CriteriaField = @criteria)
 
Note that the WRITE mutator can only be used on NON-NULL fields. 
In fact this can also be used to do a full update (if the column does not contain NULL), by setting @offset to 0 and @length to NULL (or to the actual length), as in the following example: 
DECLARE @tmp VARBINARY(MAX) --Change to the correct datatype here
SELECT @tmp = BulkColumn FROM OPENROWSET (BULK 'C:\Test\Test1.pdf', SINGLE_BLOB) a
UPDATE MyTable SET BlobField .WRITE (@tmp, 0, NULL) WHERE (CriteriaField = @criteria)

为什么hibernate为了加载@OneToOne双向关联而执行两个查询？(Why hibernate perform two queries for eager load a @OneToOne bidirectional association?)

最满意答案

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