java - spring-data postgres transaction isolation issue -

i have 2 entities: message , session. message has @manytoone relationship session.

i have spring data repository , action a:

@query("select c messages c c.session.mode=0 , c.field=5") list<messages> findmessages(); 

then process found data

messages.foreach(message->{     session session = message.getsession();     sessionclose(session);     newsessionopen(); }) 

and in other service class have

session session=findopenedsession(); 

the question is:

what if action in transaction , after starts , before ends other service request opened session or try insert record message table?

in other words have:

1. transaction starts
2. records being read
3. session instances processed in loop - session using sessionclose , newsessionopen
4. transaction ends

what if other process request opened session between 2 , 4 or 2 , 3 or somewhere within? opened session read? old 1 or new one? use postgres , @transactional spring annotation.

assuming 'session' in question refers entity created.

according this: https://www.postgresql.org/docs/9.1/static/transaction-iso.html default isolation level of postgres prevents dirty reads. no other transaction able see changes made in first transaction, until transaction commits (i.e. ends).

according same source can set transaction isolation level read uncommited. in such scenario second transaction might able see changes done first transaction, although first transaction not yet commited. note though changes entities not imediately written database, flushed before end of transaction, explicite call flush or if configured, before selects executed. although database settings might allow dirty reads might not experience them due behavior of jpa/hibernate