Wrong index picked

Question

Why did sometimes SAP choose a wrong index for a table?
We were creating reports that takes data from DFKKOP and IST_TDATA tables. These table contains millions of data. E.g. in one month, the table DFKKOP grows from 0 records to 12 millions of records. So index takes very important part here.
We had created additional indexes to improve query performance, but somehow SAP ignores these indexes and use full table scan. We had tried several methods, i.e:
- Fields order in WHERE clause is the same as in the index.
- Using Oracle HINTS INDEX .
- Using Oracle HINTS FIRST_ROWS.
- Update table statistic
- Adjusting the index (SE14)
Sometimes the right index can be chosen by using HINTS INDEX, but sometimes we have to use the FIRST_ROWS hints, and sometimes none can give the right index. In a unique case I have a query with 1-2 records in FOR ALL ENTRIES internal table that SAP can use the right index. But when the internal table in FOR ALL ENTRIES contains 3 or more records,  the same query use full table scan which is extremely slow.
We are using ECC 6 and Oracle 10g.
Please help... Is there notes or fix that I should implement?

Answer 1

About the last unique problem, here is the code:
  data:
    begin of it_vkont occurs 100,
      gpart like fkkvkp-gpart,
      vkont like fkkvkp-vkont,
    end of it_vkont,
    begin of it_object_id occurs 0,
      vkont like ist_tdata-vkont,
      object_id like ist_tdata-object_id,
    end of it_object_id.
* -- for snote contains 3 records, this query will return 3 records 
  select distinct vkont object_id into table it_object_id from ist_tdata
    where object_id in snotel
    %_hints oracle 'FIRST_ROWS'.
  if it_object_id[] is initial.
    message e000(0k) with 'Phone number does not exist'.
    return.
  endif.
* -- for snote contains 3 records, this query will also return 3 records 
  select gpart vkont appending table it_vkont from fkkvkp
    for all entries in it_object_id
    where vkont = it_object_id-vkont.
  if it_vkont[] is initial.
    message e000(0k) with 'Contract Account does not exist'.
    return.
  endif.
* -- for snote contains 3 (or more) records, SAP will choose a wrong index
  select distinct gpart vkont opbel opupk
         bukrs blart waers budat
         bldat betrw hvorg tvorg
  into table t_data
  from dfkkop
  for all entries in it_vkont
  where gpart = it_vkont-gpart
    and faedn in sfaedn
    and waers in swaers
    and applk eq 'T'
    and bukrs eq '1000'
    and augst eq space
    and augbl eq space
    and ( tvorg ne '0500' or tvorg ne '0420' )
    and blart in ('IP', 'IF', 'IN', 'YY')
    %_hints oracle 'INDEX(DFKKOP"Z04")'.
  sort t_data by gpart vkont opbel opupk.
For workaround I have replaced the last query to a bad query performance practice but works pretty well in our case.
  refresh t_data.
  loop at it_vkont.
    select distinct gpart vkont opbel opupk
           bukrs blart waers budat
           bldat betrw hvorg tvorg
    appending table t_data
    from dfkkop
    where gpart = it_vkont-gpart
      and faedn in sfaedn
      and waers in swaers
      and applk eq 'T'
      and bukrs eq '1000'
      and augst eq space
      and augbl eq space
      and ( tvorg ne '0500' or tvorg ne '0420' )
      and blart in ('IP', 'IF', 'IN', 'YY')
      %_hints oracle 'INDEX(DFKKOP"Z04")'.
  endloop.
  sort t_data by gpart vkont opbel opupk.
  delete adjacent duplicates from t_data.
The index Z04 in DFKKOP contains these ordered fields :
    GPART     Business Partner Number
    FAEDN     Due date for net payment
    WAERS     Transaction Currency
    APPLK     Application area
    BUKRS     Company Code
    AUGST     Clearing status
    AUGBL     Clearing Document or Printed Document
    TVORG     Subtransaction for Document Item
    BLART     Document Type