11:10 p.m.
Difference between "left join" and "left join fetch"
----------------------------------------------------
Key: HHH-3875
URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-3875
Project: Hibernate Core
Issue Type: Task
Reporter: Sandeep Vaid
Priority: Minor
What is the difference between the following 2 queries:
from Product product left join product.productNamesList where product.productId =
'1'
from Product product left join fetch product.productNamesList where
product.productId = '1'
I know one advantage of "left join" is that i can alias the joined
association and use this alias in with clause as:
from Product product left join product.productNamesList pnList with pnList.name like
"%a%" where product.productId = '1'
BUT this is not possible with "left join fetch".
Can we say that "left join fetch" COMPULSORY means FETCH ALL productNames for
this product?
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6:50 p.m.
New subject: [Hibernate-JIRA] Commented: (HHH-3875) Difference between "left join" and "left join fetch"
[
http://opensource.atlassian.com/projects/hibernate/browse/HHH-3875?page=c...
]
Sandeep Vaid commented on HHH-3875:
-----------------------------------
"Left join" --->
Query q = session.createQuery("from Item i join i.bids b");
Iterator pairs = q.list().iterator();
while ( pairs.hasNext() ) {
Object[] pair = (Object[]) pairs.next();
Item item = (Item) pair[0];
Bid bid = (Bid) pair[1];
}
Instead of a List of Items, this query returns a List of Object[] arrays. At index 0 is
the Item, and at index 1 is the Bid. A particular Item may appear multiple times, once for
each associated Bid.
These duplicate items are duplicate in-memory references, not duplicate instances!
2. "left join fetch" ---> This is also known as eager dynamic fetching.
from Item i
left join fetch i.bids
where i.description like '%Foo%'
When executed, it returns a list of Item instances, with their bids collections fully
initialized. This is quite different if you compare it to the ordered pairs returned by
the queries in the previous section!
The corresponding generated SQL will be:
select i.DESCRIPTION, i.INITIAL_PRICE, ...
b.BID_ID, b.AMOUNT, b.ITEM_ID, b.CREATED_ON
from ITEM i
left outer join BID b on i.ITEM_ID = b.ITEM_ID
where i.DESCRIPTION like '%Foo%'
An additional WITH clause wouldn't make sense here. You can't restrict the Bid
instances: All the collections must be fully initialized.
Personally I don't agree with author and hibernate team that WITH clause doesn't
make any sense here... Why I am bound to get all the collections? What if I want only
those bids whose
Amount >100 ?
Difference between "left join" and "left join
fetch"
----------------------------------------------------
Key: HHH-3875
URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-3875
Project: Hibernate Core
Issue Type: Task
Reporter: Sandeep Vaid
Priority: Minor
What is the difference between the following 2 queries:
from Product product left join product.productNamesList where product.productId =
'1'
from Product product left join fetch product.productNamesList where
product.productId = '1'
I know one advantage of "left join" is that i can alias the joined
association and use this alias in with clause as:
from Product product left join product.productNamesList pnList with pnList.name
like "%a%" where product.productId = '1'
BUT this is not possible with "left join fetch".
Can we say that "left join fetch" COMPULSORY means FETCH ALL productNames
for this product?
--
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7:31 a.m.
New subject: [Hibernate-JIRA] (HHH-3875) Difference between "left join" and "left join fetch"
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2 comments
2 participants
participants (2)
-
Sandeep Vaid (JIRA) -
Steve Ebersole (JIRA)