Is there any benefit of using #NamedQuery over #NamedNativeQuery in hibernate or vice verse.
I cant spot the exact difference or
in which context we should use #NamedQuery over #NamedNativeQuery
Thanks in advance.
#NamedNativeQuery lets you write a named SQL query, while #NamedQuery lets you write a named HQL query (or JPQL).
In general, you should prefer to write HQL queries because then you can let Hibernate handle the intricacies of converting the HQL into the various SQL dialects. This will make your job much simpler when you choose to switch DBMS providers.
When taking about performance, you have to know something about what goes on under the hood.
You have probably programmed something using straightforward JDBC, so you know how to queries
get passed to the driver and send to the database. When using HQL or JPA-QL the queries first have
to be parsed into an SQL language that the database can understand. In this case, we have an extra
parsing step in between. Note that Native SQL queries, including stored procedure calls, the persistence
framework still takes care of mapping the JDBC result sets to graphs of persistent objects.
If you want to include a native SQL hint to instruct the database management systems query optimizer,
for example, you need to write the SQL yourself. HQL and JPA-QL do not have keywords for this.
The disadvantage of putting native SQL in your mapping metadata is lost database portability, because
your mappings, and hence your application, will work only for a particular database. But usually this is of a
minor concern as you are probably not creating a framework that has to work on every database.
When you want to get behind the performance of your query, you really have to consult the database
and look at the execution plan - A DBA can tell you exactly what is good and what can be optimized.
#NamedQuery should be constructed with the query language (HQL or persistence query language). #NamedNativeQuery should be constructed with native SQL.
Related
Are sql queries specified inside #NamedNativeQuery pre-compiled just like #NamedQuery in JPA? I am asking this because I couldn't find anything stating it is or it isn't.
#NamedQuery "pre-compilation" is basically translating in advance to the native query language (typically SQL), so you can do it just once at application start / first use and not every time you issue the query.
#NamedNativeQuery queries are written in the native query language already, so in this sense, they are "intrinsically" precompiled.
The amount of pre-processing done to the queries annotated with #NamedNativeQuery is dependent on the JPA Provider, however you shouldn't assume much is happening, since the query is native to the underlying database, so nothing is happening at the JPA level. This is especially the case if you are calling stored procedures or something very database specific JPA is not aware of. There is no translation from JPQL to SQL.
What might go on underneath the hood is some optimisations around prepared statements for those named queries. But it depends on the JPA provider and its level of interaction with the JDBC driver of your specific database.
While joining multiple tables on my project using Hibernate jpa /Spring (annotation driven), I had to use the NamedNativeQuery annotation to achieve my objective to extract a distributed resultset spanning multiple tables. This may be a question that merely serves academic merit, but given that I am starting out on Hibernate - is there another way to achieve table joins without having to fall back on queries native to the database dialect?
Yes. I believe this is exactly what you need: https://docs.jboss.org/hibernate/entitymanager/3.5/reference/en/html/querycriteria.html#querycriteria-tuple
Criteria Queries is a way of building a complete query just using its API. If I were you, I'd give it a try.
By the way, according to your question, the reason for using native queries is just for retrieving a specific set of columns. If this is the case, you can also write it using HQL as well. The query doesn't necessarily needs to be native.
How many ways Hibernate provide to access database?
For example, I want to CRUD an object to database, I found out:
Using session from SessionFactory:
session.save(object);
...
Using Hibernate Query Language.
Using Hibernate Criteria Queries.
Using Native SQL.
But I don't know what I should use. Please list your practice to access database in PRIORITY DECREASING ORDER and the reason why you do that.
Thank you.
If you have an ID and wants the associated entity, the use Session.get(). It's efficient, and makes use of the first-level cache to avoid reexecuting the query again and again.
If you need to get entities via other criteria (like all the users with a given first name, for example), then use JPQL queries. They are simple to write, very readable, and have less limitations than criteria queries.
If you need to take various optional criteria (like for a complex search form), the criteria API is the tool for the job. But it can't do everything a JPQL query does. There are other APIs available, and you can relatively easily write an API that generates dynamic JPQL queries if needed.
If you have a really complex query that can't be expressed using JPQL, then use SQL.
To write things to the database, queries should generally not be used, except in very specific circumstances where many entities must be modified the same way. Instead, get the entities to modify, and modify them. Hibernate will save their new state automatically.
I am working in a project which uses JPA ORM and framework provides two kinds of method to create queries.
entityManager.createQuery(query1);
entityManager.createNativeQuery(query2);
I understand the kinds of query string is to be passed to use them, but I don't know exactly why do we need to create native query? Probably we don't want to use ORM capabilities there?
You do not need to create a native query unless you want to. JPQL eventually is translated into SQL by the framework but the framework lets you call the native query also. Why would want to do that:
Low level access, which means that you can optimize and handle the mapping by yourself; with SQL you actually access the database table while with JPQL you access the entity objects;
Maybe you do not want to learn JPQL if you already know SQL
You already have the queries written in SQL, and do not have resources/time to port them to JPQL
createQuery uses JPAs own query language, you select from Class names instead of table names. This is not SQL, it is just similar, and is later transformed to real SQL. Mapping to java classes will be done automatically and actual class instances will be returned as result.
createNativeQuery uses real SQL, and will not be able to use JPA features. This method is used in general if you need to do something really odd that is not supported by JPA. A list of Object[] will be returned, and mapping to java objects will have to be done manually. In other words, its just like working with a DB before JPA came to, just slightly more convenient since connection handling is done automatically.
I have used it for optimization purposes. Using Native queries means that the ORM mapping is not in place, and instead of JPQL, you use the DB's native syntax. So, as #RasmusFranke also pointed out, if you need something that is not supported by JPA (like when you want to use DB vendor specific extensions, which is conceptually a bad idea, since JPA is all about being DB agnostic, but happens nevertheless. I know...)
The other effect of this is that by using native queries, only the supplied query is run. No eager fetching of other entities, or other unwanted stuff. This way, if you deal with huge amounts of objects, you can save some heap space.
I (think I) understand that Hibernate offers me access to a relational database essentially as if I had a class(es) that matched whatever view(s) I define of some tables.
How do I get the results of a specific query to be accessible via a class? In particular, can I issue a complex SQL query, and process multiple results?
Do I lose any of the power of SQL by using Hibernate?
I (think I) understand that Hibernate offers me access to a relational database essentially as if I had a class(es) that matched whatever view(s) I define of some tables.
Hibernate provides a framework allowing to map an object model to your database and an API to manipulate data through this object model.
How do I get the results of a specific query to be accessible via a class? In particular, can I issue a complex SQL query, and process multiple results?
I'm not sure I understood the question but let's see. The Hibernate way would be to use HQL (Hibernate Query Language) queries and/or Criteria queries to perform queries on the object model.
But you can also use Native SQL (sacrificing portability) to return entities, multiple entities or even non-managed entities (see also Hibernate 3.2: Transformers for HQL and SQL).
Do I lose any of the power of SQL by using Hibernate?
HQL and the Criteria API are quite powerful - and portable - and will generate the proper SQL for your backend. If required, you can still use native SQL queries as already mentioned. But in most cases, HQL and Criteria work well and should be preferred.
See also
14. HQL: The Hibernate Query Language
15. Criteria Queries
16. Native SQL
Hibernate3 allows you to specify handwritten SQL, including stored procedures, for all create, update, delete, and load operations.
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").list();
More at http://docs.jboss.org/hibernate/core/3.3/reference/en/html/querysql.html