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JPA Hibernate query vs Native query

I use Spring Data JPA (hibernate) generated queries for fetching data from my Sqlserver. Now i am getting performance related issues in my system.
Load findByLoadId(Integer loadId);
This is the query i am using to get data. This query returns 25 cell data but i only use 5 data from that.
can i use direct native query like
select id,date,createdBy,createdOn,loadName from Load where
loadId=:loadId
but if native query is suggestable then I am having question like Does ORM frameWork reduce performence by getting unneeded data from Database?

By "data cell" I assume that you are referring to database table columns, and not to records. The answer to your question is that yes, ORM frameworks might tend to just do a SELECT * under the hood, which can result in unwanted information being sent across the network to your application. If the JPA repository interface is behaving this way, you may switch to either an explicit JPA query (e.g. using the #Query annotation), or even a native query. Then, just select the columns you want. The issue here is that ORM frameworks map object templates (e.g. classes) to entire database tables. So, the concept of entity implicitly includes every database column. If you go with the option of selecting only certain columns, you may need to do some juggling on the Java side. Note that if the use a JPA query, your code would still, in theory, be database independent.

Inserting data in one table using HQL in Hibernate

I was reading the Hibernate HQL tutorial and found that HQL doesn't support INSERT INTO..VALUES.. but INSERT INTO..SELECT.. i.e. HQL only support insert from another table.
Suppose I want to insert same values in one table and that data is not from any other table i.e. the values are not in any other table.Then how can I do that in HQL?
Also, would like to know the rational behind such restrictions in HQL?

You don't need to use hql to insert if the data is from another table.
Simply get a reference to your entity, get a hold of a Hibernate session, and call save().
According to http://docs.jboss.org/hibernate/orm/4.0/devguide/en-US/html/ch04.html#d0e2116
Pseudo-syntax for INSERT statements
INSERT INTO EntityName properties_list select_statement
Only the INSERT INTO ... SELECT ... form is supported. You cannot specify explicit values to insert.

Hibernate is an ORM framework (Object-Relational Mapping).
Its job is that you give objects (Entities) to it and he manages the storage (through Session.save(), IIRC).
So, you do not use the HQL to insert new records, but use the ORM methods.
And (this is a guess) on the other hand, since loading entities from a table, copying them to other entities and storing them one by one is slow, HQL provides a shortcut to the SQL in the DB just for that specific operation for performance purposes.

You can use session.save(object) to insert data into tables.

jdbc code changes to maintain connection

I am a newbie and have some question on using jdbc with java:
What changes in the code I will have to make to:
change database type? (i.e. from PostgreSQL to MySQL)
use the table in the code after I decide to drop one of the colums from that table.
Also - how to make queries cached?

change database type? (i.e. from PostgreSQL na MySQL)
Replace the MySQL JDBC driver in the classpath with the PostgreSQL JDBC driver. Update the JDBC connection URL to point to PostgreSQL DB instead of MySQL DB. If necessary, also update your SQL queries to replace any MySQL-specific SQL functions/clauses by PostgreSQL-specific ones.
use the table in the code after I decide to drop one of the colums from that table.
Remove the column in question from the SQL queries. If necessary, also update the entity (the custom Javabean class which you have to represent one row of the DB) to remove the property and getter/setter.
Also - how to make queries cached?
Use PreparedStatement instead of Statement. If possible, replace all the JDBC code by a fullworthy ORM such as JPA or good ol' Hibernate. They not only minimizes JDBC boilerplate code to oneliners, but they also offers second-level caching capabilities as well.

State of the database when mixing HQL with native SQL in Hibernate

In my database code I use some Hibernate native SQL queries (inserts, deletion, updates). I understand that when I use HQL and the cache is on than the state of the DB is stable whenever I call the DB with the HQL. However, I wonder what happens if I use native SQL queries, e.g. I insert some data (but do not commit it) and than I try to fetch some data with a HQL query. Will I get the inserted data too?
Any hints?

I would say, that it depends on underlying database and transactions setting. But even HQL is translated to native SQL and executed. As long as you stay in same transaction, you will be able to load changes made with native SQL via HQL. But keep in mind, that HQL queries interat with caches, proxies and other hibernate stuff - there could be some weird issues, because natiev SQL completely bypasses this ( this is purpose of native queries - fast lane around all the hibernate stuff )

Adding to Konstantin Pribluda's answer I can say that in the reverse situation : adding data through Hibernate(even with session.save()) and then fetching data with native SQL resulted in the native SQL-query not fetching the added data.
So when using different types of query's in the DAO, I always flush the session first before I use a native SQL query... Never know which method's get mixed in the Service layer.

Yes, you will get it because all of your DAO queries will executed one by one whether its HQL or SQL. So if insert query is first in order then you will get inserted records.

SQL , insert is possible on the table which is not present in terms of hibernate

does insert query in hibernate requires table to be present. I mean when i have a query that insert some values in to the table which is not present . will the hibernate gives the exception or it will through the exception only when update is done .
I'm speaking in terms of only hibernate .
Can you please suggest

A table being present in terms of Hibernate doesn't make much sense in my opinion. A table is present in the database, or not. If it is not and if you have an Entity mapped on it, trying to persist this Entity will fail at flush time, when Hibernate will perform the SQL insert, and you'll get a HibernateException wrapping a JDBC exception.
If this doesn't answer the question, please clarify, I did not understand the part where you're referring to an update.

I think you're asking if the table is missing will Hibernate throw the error or will the database throw the error.
IIRC, I don't think Hibernate will even initialize if the table is missing. You will get an error when its going through the mapping files and trying to pre-generate the CRUD SQL statements based on the datatypes in the database.