Why does queryset.iterator() show poor performance












0















# I am using postgresql
# queryset run complex SQL joining 4 tables

print(queryset.count()) # print 30000

# it takes 5 sec
for i in queryset:
print(i.arg)

# it takes 10 sec
for i in queryset.iterator(chunk_size=30000):
print(i.arg)


I simplify situation I have suffered like above.



iterator() takes more time about 2 times.



Of course, I understand that iterator() need more DB-side operation and more request like DECLARE, FETCH and CLOSE because it is using DB CURSOR internally(in practice I check for Django to emit these request through DB log).



However, I don't think these additional operations will take that much time. It even will fetch all rows at one FETCH request since chunk_size is the same as the total number of rows.



I am wondering if this situation is expected one or not. If it is expected, why it takes that much more time?










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    0















    # I am using postgresql
    # queryset run complex SQL joining 4 tables

    print(queryset.count()) # print 30000

    # it takes 5 sec
    for i in queryset:
    print(i.arg)

    # it takes 10 sec
    for i in queryset.iterator(chunk_size=30000):
    print(i.arg)


    I simplify situation I have suffered like above.



    iterator() takes more time about 2 times.



    Of course, I understand that iterator() need more DB-side operation and more request like DECLARE, FETCH and CLOSE because it is using DB CURSOR internally(in practice I check for Django to emit these request through DB log).



    However, I don't think these additional operations will take that much time. It even will fetch all rows at one FETCH request since chunk_size is the same as the total number of rows.



    I am wondering if this situation is expected one or not. If it is expected, why it takes that much more time?










    share|improve this question

























      0












      0








      0


      1






      # I am using postgresql
      # queryset run complex SQL joining 4 tables

      print(queryset.count()) # print 30000

      # it takes 5 sec
      for i in queryset:
      print(i.arg)

      # it takes 10 sec
      for i in queryset.iterator(chunk_size=30000):
      print(i.arg)


      I simplify situation I have suffered like above.



      iterator() takes more time about 2 times.



      Of course, I understand that iterator() need more DB-side operation and more request like DECLARE, FETCH and CLOSE because it is using DB CURSOR internally(in practice I check for Django to emit these request through DB log).



      However, I don't think these additional operations will take that much time. It even will fetch all rows at one FETCH request since chunk_size is the same as the total number of rows.



      I am wondering if this situation is expected one or not. If it is expected, why it takes that much more time?










      share|improve this question














      # I am using postgresql
      # queryset run complex SQL joining 4 tables

      print(queryset.count()) # print 30000

      # it takes 5 sec
      for i in queryset:
      print(i.arg)

      # it takes 10 sec
      for i in queryset.iterator(chunk_size=30000):
      print(i.arg)


      I simplify situation I have suffered like above.



      iterator() takes more time about 2 times.



      Of course, I understand that iterator() need more DB-side operation and more request like DECLARE, FETCH and CLOSE because it is using DB CURSOR internally(in practice I check for Django to emit these request through DB log).



      However, I don't think these additional operations will take that much time. It even will fetch all rows at one FETCH request since chunk_size is the same as the total number of rows.



      I am wondering if this situation is expected one or not. If it is expected, why it takes that much more time?







      python django postgresql-9.5






      share|improve this question













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      asked Dec 29 '18 at 15:56









      SangminKimSangminKim

      2,33442360




      2,33442360
























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