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.
Related
After making some search on the web, I think that when using Spring Data JPA Named Queries, we need some extra implementation or definitions comparing to the derived or dynamic queries in Spring Data JPA. In this scene, I am really wondering that do we really need to use Spring Data JPA Named Queries?
Spring Data derived queries are intended (and useful) only for very simple queries. Those queries where you look at the name that you would naturally give such a method and would immediately know how to implement it in SQL or JPQL.
As soon as a query gets a little more complex we shouldn't use derived queries anymore, and often we can't even if we wanted to. For example query derivation doesn't have a way to control the precedence between AND and OR.
For all other queries we need to explicitly code the query one way or the others. And if you don't want your queries mixed with your repository, a named query is a very viable alternative.
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.
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.
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