Hibernate - HQL pagination

user_752462 0
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This is a problem similar to: HQL - row identifier for pagination

I'm trying to implement pagination using HQL. I have a PostgreSQL database.

int elementsPerBlock = 10;
int page = 2; //offset = 2*10

String sqlQuery = "FROM Messages AS msg " +
                  " LEFT JOIN FETCH msg.commands AS cmd " +   
                  "ORDER BY msg.identifier ASC" ;

Query query = session.createQuery( sqlQuery )
                     .setFirstResult( elementsPerBlock * ( (page-1) +1 ) )
                     .setMaxResults( elementsPerBlock );

What happens is that Hibernate fetches ALL the Messages, and returns the needed ones after they were all loaded.

Thus, Hibernate fetches 210000 entities instead of the 30 which are returned (each Messages has exactly 2 commands).

Is there a way to reduce the overhead by a factor of 7000?

edit: I've tries adding .setFetchSize( elementsPerBlock ) . It didn't help.

edit 2: the SQL query that is generated is:

select ... 
from schemaName.messages messages0_ 
left outer join schemaName.send_commands commands1_ 
on messages0_.unique_key=commands1_.message_key 
order by messages0_.unique_identifier ASC

Absolutenly no LIMIT or OFFSET

hql hibernate pagination
6 Answers

I think your original exception is not correct.

What happens is that Hibernate fetches ALL the Messages, and >returns the needed ones after they were all loaded.

What happens while query processing is that setFirstResult(calculateOffset(page, limitPerPage)) gets translated to OFFSET and setMaxResults(limitPerPage) gets translated to LIMIT

user_96108930
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