if (mssql)
{
    return mssqlQuery;
}
else
{
    return postgresQuery;
}

interface UserDao{
  User GetSomeUser();
}

class MssqlUserDao : UserDao {
  User GetSomeUser()
  {
     return new User ...(mssql.query("SELECT TOP 1..."));
  }
}
class PsqlUserDao : UserDao {
  User GetSomeUser()
  {
     return new User ...(psql.query("SELECT... LIMIT 1"));
  }
}

public class FooDto
{
    public  Id {get;}
    public  A {get;set;}
    public  B {get;set;}
}


public interface IFooRepository
{
     IEnumerable<FooDto>  GetItems( params );
     void UpdateItem(FooDto item);     
}

public MsSqlRepository : IFooRepository
{
    //... специфика работы с MsSql
}

public PostgresRepository : IFooRepository
{
    //... специфика работы с Postgres
}

public static Dependencies
{

    // делаешь любым удобным способом переключение с MsSql реализации на Postgres
    public static IFooRepository FooRepository =>  new MsSqlRepository(connectionString);
                                                   //new PostgresRepository(connectionString);
}

// далее работаешь с FooRepository через интерфейс
Dependencies.FooRepository.GetItems();

SELECT /*+ LEADING(e2 e1) USE_NL(e1) INDEX(e1 emp_emp_id_pk) 
           USE_MERGE(j) FULL(j) */
    e1.first_name, e1.last_name, j.job_id, sum(e2.salary) total_sal
  FROM employees e1, employees e2, job_history j
  WHERE e1.employee_id = e2.manager_id
    AND e1.employee_id = j.employee_id
    AND e1.hire_date = j.start_date
  GROUP BY e1.first_name, e1.last_name, j.job_id
  ORDER BY total_sal;

O>SELECT /*+ LEADING(e2 e1) USE_NL(e1) INDEX(e1 emp_emp_id_pk) 
O>           USE_MERGE(j) FULL(j) */
O>    e1.first_name, e1.last_name, j.job_id, sum(e2.salary) total_sal
O>  FROM employees e1, employees e2, job_history j
O>  WHERE e1.employee_id = e2.manager_id
O>    AND e1.employee_id = j.employee_id
O>    AND e1.hire_date = j.start_date
O>  GROUP BY e1.first_name, e1.last_name, j.job_id
O>  ORDER BY total_sal;
O>

using (var db = new DataConnection())
{
   db.StatementHints.Add(@"/*+ LEADING(e2 e1) USE_NL(e1) INDEX(e1 emp_emp_id_pk) 
           USE_MERGE(j) FULL(j) */");

   var query = from e1 in db.GetTable<Employee>()
               from e2 in db.GetTable<Employee>().InnerJoin(e2 => e2.ManagerId == e1.EmployeeId)
               from j  in db.GetTable<JobHitory>().InnerJoin(j => j.EmployeeId == e1.EmployeeId && j.StartDate == e1.HireDate)
               group new {e1, e2} by new { e1.FirstName, e1.LastName, j.JobId } into g
               select new 
               {
                  g.Key.FirstName,    
                  g.Key.LastName,    
                  g.Key.JobId,
                  TotalSal = g.Sum(e => e.e2.Salary)
               };
    query = query.OrderBy(e => e. TotalSal);
    var result = query.ToList();
}

D>using (var db = new DataConnection())
D>{
D>   db.StatementHints.Add(@"/*+ LEADING(e2 e1) USE_NL(e1) INDEX(e1 emp_emp_id_pk) 
D>           USE_MERGE(j) FULL(j) */");
D>}
D>

select schema.sequence.NEXTVAL from dual;