Performance Problem with Nested Select

Question

I have an urgent performance question, I run a nested select like this one
  SELECT tabname
           INTO ls_dd02l
           FROM dd02l
           WHERE tabname LIKE c_feld
           AND   buffered IS NOT NULL.
      SELECT tabname fieldname keyflag position
             INTO CORRESPONDING FIELDS OF ls_dd03l
             FROM dd03l
             WHERE tabname   = ls_dd02l-tabname
             AND   fieldname IN ('MANDT', 'CLIENT', 'CLNT')
             AND   keyflag   = 'X'.
        IF ( sy-subrc EQ 0 ).
          wa-tabname   = ls_dd03l-tabname.
          wa-fieldname = ls_dd03l-fieldname.
          wa-keyflag   = ls_dd03l-keyflag.
          wa-position  = ls_dd03l-position.
          APPEND wa TO itab.
        ENDIF.
      ENDSELECT.
    ENDSELECT.
It is taking about 20sec, which is much too long.
How can I improve the performance.
Points rewarded!
S.B.

Answer 1

Hi Siegfried,
As Vinay said  u can use INNER JOIN to get the data from both database tables
or u can use FOR ALL ENTRIES statement.
You can use the option FOR ALL ENTRIES to replace nested select loops by operations on internal tables. This can significantly improve the performance for large sets of selected data.
The above statement is from Library. Please check this link
http://help.sap.com/saphelp_nw04/helpdata/en/fc/eb3a1f358411d1829f0000e829fbfe/content.htm
Tabular Conditions sub heading
SELECT a~tabname
       b~fieldname
       b~keyflag
       b~position
    FROM dd02l as a INNER JOIN dd03l as b
      ON a~tabname = b~tabname
     INTO TABLE itab
   WHERE a~tabname LIKE c_feld
     AND buffered IS NOT NULL
     AND fieldname IN ('MANDT', 'CLIENT', 'CLNT')
     AND keyflag   = 'X'.
OR
SELECT tabname
       INTO TABLE lt_dd02l
       FROM dd02l
       WHERE tabname LIKE c_feld
       AND   buffered IS NOT NULL.
IF NOT lt_dd02l IS INITIAL.
      SELECT tabname fieldname keyflag position
             INTO CORRESPONDING FIELDS OF TABLE itab
             FROM dd03l
             FOR ALL ENTRIES IN lt_dd02l
             WHERE tabname   = lt_dd02l-tabname
             AND   fieldname IN ('MANDT', 'CLIENT', 'CLNT')
             AND   keyflag   = 'X'.
ENDIF.
Please check this link
/people/rob.burbank/blog/2007/03/19/joins-vs-for-all-entries--which-performs-better
You have to look at the primary keys..
MANDT
STLTY
STLNR
STLAL
STLKN
STASZ
MANDT
STLTY
STLNR
STLKN
STPOZ
You need the stlty for both tables stas and stpo
The on condition between stas and stpo is obvious, will help a
bit.
It would be much better, if you could add it to the WHERE Condition.
Do you know it?
If not, then as the BOM categories are not so many,
Try
select distinct stlty
from stas.
and add the values to you where condition as
AND s1-stlty in ( ....... )
This should improve the performance.
I don't kknow whether the BOM Categories can change in future.
instead of a fixed in, you could add a subselect from a customizing table.
I am quite confident that the solution I proposed will be much faster than the FOR ALL ENTRIES.
But with more complicated joins you really have to try it.
There is no general rule about the number of tables in a join, there are perfectly running joins with 5 tables
and there are porblems with 2 tables.
You must analyze the indexes, it it is clear which index will support the accesses then it will work fine.
The FOR ALL ENTRIES is by construction much slower!
Best regards,
raam