Hibernate Delete query

Question

When I try to delete an entry from a db  using   session delete object     then I can the following   1  If the row is present in DB then two SQL queries are getting executed  A select and then a delete  2  If the row is not present in the DB then only the select query is getting executed  But again this is not the case for update  Irrespective of the presence of DB row  only the update query is getting executed   Please let me know why this kind of behaviour for delete operation  Isn t it a performance issue since two queries are getting hit rather than one   Edit    I am using hibernate 3 2 5  Sample code   SessionFactory sessionFactory   new Configuration   configure  student cfg xml   buildSessionFactory        Session session   sessionFactory openSession        Student student   new Student        student setFirstName  AAA        student setLastName  BBB        student setCity  CCC        student setState  DDD        student setCountry  EEE        student setId  FFF        session delete student       session flush                session close      cfg xml   lt property name  hibernate connection username  gt system lt  property gt       lt property name  hibernate connection password  gt XXX lt  property gt       lt property name  hibernate connection driver class  gt oracle jdbc OracleDriver lt  property gt       lt property name  hibernate connection url  gt jdbc oracle thin  localhost 1521 orcl lt  property gt             lt property name  hibernate jdbc batch size  gt 30 lt  property gt       lt property name  hibernate dialect  gt org hibernate dialect OracleDialect lt  property gt       lt property name  hibernate cache use query cache  gt false lt  property gt       lt property name  hibernate cache use second level cache  gt false lt  property gt       lt property name  hibernate connection release mode  gt after transaction lt  property gt       lt property name  hibernate connection autocommit  gt true lt  property gt       lt property name  hibernate connection pool size  gt 0 lt  property gt       lt property name  hibernate current session context class  gt thread lt  property gt           lt property name  hibernate show sql  gt true lt  property gt       lt property name  hibernate hbm2ddl auto  gt update lt  property gt            hbm xml   lt  xml version  1 0  encoding  UTF-8   gt   lt  DOCTYPE hibernate-mapping PUBLIC  -  Hibernate Hibernate Mapping DTD 3 0  EN   http   hibernate sourceforge net hibernate-mapping-3 0 dtd  gt   lt hibernate-mapping gt   lt class name  com infy model Student  table  STUDENT  gt       lt id name  id  column  ID  gt           lt generator class  assigned  gt  lt  generator gt       lt  id gt       lt property name  firstName  type  string  column  FIRSTNAME  gt  lt  property gt       lt property name  lastName  type  string  column  LASTNAME  gt  lt  property gt       lt property name  city  type  string  column  CITY  gt  lt  property gt       lt property name  state  type  string  column  STATE  gt  lt  property gt       lt property name  country  type  string  column  COUNTRY  gt  lt  property gt           lt  class gt

User · Answer

I m not sure but    If you call the delete method with a non transient object  this means first fetched the object from the DB  So it is normal to see a select statement  Perhaps in the end you see 2 select   1 delete  If you call the delete method with a transient object  then it is possible that you have a cascade  delete  or something similar which requires to retrieve first the object so that  nested actions  can be performed if it is required      Edit  Calling delete   with a transient instance means doing something like that   MyEntity entity   new MyEntity    entity setId 1234   session delete entity     This will delete the row with id 1234  even if the object is a simple pojo not retrieved by Hibernate  not present in its session cache  not managed at all by Hibernate    If you have an entity association Hibernate probably have to fetch the full entity so that it knows if the delete should be cascaded to associated entities

User · Answer

To understand this peculiar behavior of hibernate  it is important to understand a few hibernate concepts -   Hibernate Object States      Transient - An object is in transient status if it has been   instantiated and is still not associated with a Hibernate session       Persistent - A persistent instance has a representation in the   database and an identifier value  It might just have been saved or   loaded  however  it is by definition in the scope of a Session       Detached - A detached instance is an object that has been persistent    but its Session has been closed    http   docs jboss org hibernate orm 3 3 reference en html objectstate html objectstate-overview  Transaction Write-Behind  The next thing to understand is   Transaction Write behind   When objects attached to a hibernate session are modified they are not immediately propagated to the database  Hibernate does this for at least two different reasons          To perform batch inserts and updates    To propagate only the last change  If an object is updated more than once  it still fires only one update statement       http   learningviacode blogspot com 2012 02 write-behind-technique-in-hibernate html  First Level Cache   Hibernate has something called  First Level Cache   Whenever you pass an object to save    update   or saveOrUpdate    and whenever you retrieve an object using load    get    list    iterate   or scroll    that object is added to the internal cache of the Session  This is where it tracks changes to various objects   Hibernate Intercepters and Object Lifecycle Listeners -   The Interceptor interface and listener callbacks from the session to the application  allow the application to inspect and or manipulate properties of a persistent object before it is saved  updated  deleted or loaded  http   docs jboss org hibernate orm 4 0 hem en-US html listeners html d0e3069    This section Updated   Cascading  Hibernate allows applications to define cascade relationships between associations  For example   cascade-delete  from parent to child association will result in deletion of all children when a parent is deleted   So  why are these important    To be able to do transaction write-behind  to be able to track multiple changes to objects  object graphs  and to be able to execute lifecycle callbacks hibernate needs to know whether the object is transient detached and it needs to have the object in it s first level cache before it makes any changes to the underlying object and associated relationships   That s why hibernate  sometimes  issues a  SELECT  statement to load the object  if it s not already loaded  in to it s first level cache before it makes changes to it    Why does hibernate issue the  SELECT  statement only sometimes   Hibernate issues a  SELECT  statement to determine what state the object is in  If the select statement returns an object  the object is in detached state and if it does not return an object  the object is in transient state    Coming to your scenario -   Delete - The  Delete  issued a SELECT statement because hibernate needs to know if the object exists in the database or not  If the object exists in the database  hibernate considers it as detached and then re-attches it to the session and processes delete lifecycle    Update - Since you are explicitly calling  Update  instead of  SaveOrUpdate   hibernate blindly assumes that the object is in detached state  re-attaches the given object to the session first level cache and processes the update lifecycle  If it turns out that the object does not exist in the database contrary to hibernate s assumption  an exception is thrown when session flushes   SaveOrUpdate - If you call  SaveOrUpdate   hibernate has to determine the state of the object  so it uses a SELECT statement to determine if the object is in Transient Detached state  If the object is in transient state  it processes the  insert  lifecycle and if the object is in detached state  it processes the  Update  lifecycle

User · Answer

instead of using      session delete object    use  getHibernateTemplate   delete object    In both place for select   query and also for delete use getHibernateTemplate    In select query you have to use DetachedCriteria or Criteria  Example for select query  List lt foo gt  fooList   new ArrayList lt foo gt     DetachedCriteria queryCriteria   DetachedCriteria forClass foo class   queryCriteria add Restrictions eq  Column name  restriction    fooList   getHibernateTemplate   findByCriteria queryCriteria     In hibernate avoid use of session here I am not sure but problem occurs just because of session use

User · Answer

The reason is that for deleting an object  Hibernate requires that the object is in persistent state  Thus  Hibernate first fetches the object  SELECT  and then removes it  DELETE    Why Hibernate needs to fetch the object first  The reason is that Hibernate interceptors might be enabled  http   docs jboss org hibernate orm 3 3 reference en html events html   and the object must be passed through these interceptors to complete its lifecycle  If rows are delete directly in the database  the interceptor won t run   On the other hand  it s possible to delete entities in one single SQL DELETE statement using bulk operations   Query q   session createQuery  delete Entity where id   X    q executeUpdate

[java] Hibernate Delete query

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