Postgres to bring list all of table fields for particular Employee row? - postgresql

Question

Taking a reference from link: Postgres to fetch the list having comma separated values, I want to write a query which somhow brings Employee email Table fields as a list for a particular Empployee. This is needed for Spring Batch to Simply match it from the Resultset and create a POJO/Model class like List emails for Employee class?
Can this be possible ?
select c.*, ce.*, string_agg(ce.email, ',') as emails
from root.employee c
full outer join root.employee_email ce
on c.employee_id = ce.employee_id
group by
c.employee_id, ce.employee_email_id
order by
c.employee_id
limit 1000
offset 0;

Answer 1

Your problem is a common one in the batch processing realm and with Spring Batch it is called "Driving Query Based ItemReaders", you can find more about that in here.
Basically you retrieve the Contacts in your reader, and in your processor you add the list of Emails to them.
#Bean(destroyMethod = "")
public JdbcCursorItemReader<Employee> employeeReader(DataSource dataSource) {
JdbcCursorItemReader<Employee> ItemReader = new JdbcCursorItemReader<>();
ItemReader.setDataSource(dataSource);
ItemReader.setSql("SELECT * FROM employee.employee C ");
ItemReader.setRowMapper(new EmployeeRowMapper());
return ItemReader;
}
#Bean
public ItemProcessor<Employee, Employee> settlementHeaderProcessor(JdbcTemplate jdbcTemplate){
return item -> {
root.EMPLOYEE_EMAIL CE WHERE ce.employee_id = ? ",
new Object[]{item.getId()},
new RowMapper<String>() {
#Override
public String mapRow(ResultSet resultSet, int i) throws SQLException {
return resultSet.getString("EMAIL");
}
});
item.setEmails(emails);
return item;
};
}
PS : this could have some performance issues if you have lots of contacts, because for each contact Item you will hit the database to retrieve Emails.
There is another optimized way, by creating a custom reader that will return a List of Contacts (For example 1000 by 1000), and a processor that will enrich them with their emails. This way you will hit the database again for each 1000 Contact Item.
In your reader your retrieve a list of unique Employees page per page (Say your page is 1000 long).
And in your processor for the 1000 employees you retrieve all their emails in one query.
Then for each employee you set the emails retrieved in the last query.
An example might like the following:
public interface EmployeeRepository extends PagingAndSortingRepository<Employee, Integer> {
}
#Getter
class EmployeeVO {
private Long employeeId;
private String email;
EmployeeVO(Long employeeId, String email) {
this.employeeId= employeeId;
this.email = email;
}
}
public class EmployeeListReader implements ItemReader<List<Employee>> {
private final static int PAGE_SIZE = 1000;
private final EmployeeRepository employeeRepository;
private int page = 0;
public EmployeeListReader(EmployeeRepository employeeRepository) {
this.employeeRepository = employeeRepository;
}
public List<Employee> read() throws Exception {
Page<Employee> employees = employeeRepository.findAll(PageRequest.of(page, PAGE_SIZE));
page++;
return employees.getContent();
}
}
#Bean
EmployeeListReader reader(){
return new EmployeeListReader(this.employeeRepository);
}
#Bean
public ItemProcessor<List<Employee>, List<Employee>> settlementHeaderProcessor(NamedParameterJdbcTemplate namedParameterJdbcTemplate) {
return item -> {
List<Long> employeesIds = item.stream().map(Employee::getId).collect(Collectors.toList());
SqlParameterSource parameters = new MapSqlParameterSource("ids", employeesIds);
List<ContactVO> emails = namedParameterJdbcTemplate
.query("SELECT CE.employeeId, CE.EMAIL FROM employee_EMAIL CE WHERE ce.contact_id IN (:ids) ",
parameters,
new RowMapper<ContactVO>() {
#Override
public ContactVO mapRow(ResultSet resultSet, int i) throws SQLException {
return new ContactVO(
resultSet.getLong("EMPLOYEE_ID"),
resultSet.getString("EMAIL"));
}
});
Map<Long, List<ContactVO>> emailsByContactId = emails.stream().collect(Collectors.groupingBy(ContactVO::getContactId));
List<Employee> newEmployeesWithEmails = Collections.unmodifiableList(item);
newEmployeesWithEmails.forEach(employee -> {
employee.setEmails(emailsByContactId.get(employee.getId()).stream().map(ContactVO::getEmail).collect(Collectors.toList()));
});
return newEmployeesWithEmails;
};
}
Hope this helps