Performance Tips - Netsed Loops

Performance tuning is they key concept in ABAP programming. In real time, we could find large amount of data. Today we will discuss the worse effect of nested loops and what are the alternate ways to handle nested loops.
If the amount of data is large, nested loops are fully avoided due to performance issues. If your program is extracting small amount of data, then focus on SELECT statements than nested loops.
See below program where you can find three methods:
1) Nested Loop
2) Indexed Loop
3) Parallel Cursor

The indexed loop is the best method to avoid performance issues. Read statement by using binary search inside a loop works well.

The above three methods are depicted in the following program:

report ztest_nested_loop.

data: bkpf type bkpf,
     bseg type bseg.

select-options: s_bukrs for bseg-bukrs memory id buk obligatory,
               s_gjahr for bseg-gjahr memory id gjr obligatory,
               s_lifnr for bseg-lifnr memory id lif obligatory.

data: bkpf_tab type standard table of bkpf,
     bkpf_lin like line of bkpf_tab,
     bseg_tab type standard table of bseg,
     bseg_lin like line of bseg_tab.

data: start_time   type sy-uzeit,
     end_time     type sy-uzeit,
     difference   type sy-uzeit,
     bkpf_entries type sy-tabix,
     bseg_entries type sy-tabix,
     bkpf_reads   type sy-tabix,
     bseg_reads   type sy-tabix.

start-of-selection.
 perform unindexed_select.
 perform nested_loop.
 perform indexed_loop.
 PERFORM parallel_cursor.

*&---------------------------------------------------------------------*
*&      Form  unindexed_select
*&---------------------------------------------------------------------*
form unindexed_select.

 get time field start_time.

 select * from bseg
   into   table bseg_tab
   where  bukrs in s_bukrs
   and    gjahr in s_gjahr
   and    lifnr in s_lifnr.

 if sy-subrc <> 0.
   message id '00' type 'E' number '001' with 'No entries selected'.
 endif.

 select * from bkpf
   into  table bkpf_tab
   for all entries in bseg_tab
   where bukrs = bseg_tab-bukrs
   and   belnr = bseg_tab-belnr
   and   gjahr = bseg_tab-gjahr
   and   bstat = space.

 clear   bseg_tab.
 refresh bseg_tab.
 select * from bseg
   into   table bseg_tab
   for all entries in bkpf_tab
   where  bukrs = bkpf_tab-bukrs
   and    belnr = bkpf_tab-belnr
   and    gjahr = bkpf_tab-gjahr.

 get time field end_time.
 difference = end_time - start_time.
 describe table bkpf_tab lines bkpf_entries.
 describe table bseg_tab lines bseg_entries.

 write: /001 'Time for unindexed select:', difference,
        /005 'Number of BKPF entries:', bkpf_entries,
        /005 'Number of BSEG entries:', bseg_entries.
 skip 1.

endform.                    " unindexed_select

*&---------------------------------------------------------------------*
*&      Form  nested_loop
*&---------------------------------------------------------------------*
form nested_loop.

 get time field start_time.

 loop at bkpf_tab
   into bkpf_lin.
   bkpf_reads = bkpf_reads + 1.
   loop at bseg_tab
     into bseg_lin where
     bukrs = bkpf_lin-bukrs and
     belnr = bkpf_lin-belnr and
     gjahr = bkpf_lin-gjahr.
     bseg_reads = bseg_reads + 1.
   endloop.
 endloop.

 get time field end_time.
 difference = end_time - start_time.

 write: /001 'Time for nested loop:', difference,
        /005 'Number of BKPF reads:', bkpf_reads,
        /005 'Number of BSEG reads:', bseg_reads.
 skip 1.

endform.                    " nested_loop

*&---------------------------------------------------------------------*
*&      Form  indexed_loop
*&---------------------------------------------------------------------*
form indexed_loop.

 data: bkpf_index like sy-tabix,
       bseg_index like sy-tabix.

 clear: bkpf_reads,
        bseg_reads.

 get time field start_time.

 sort: bkpf_tab by bukrs belnr gjahr,
       bseg_tab by bukrs belnr gjahr.

 loop at bkpf_tab
   into bkpf_lin.
   read table bseg_tab
        into  bseg_lin
        with key
          bukrs = bkpf_lin-bukrs
          belnr = bkpf_lin-belnr
          gjahr = bkpf_lin-gjahr
        binary search.
   bkpf_reads = bkpf_reads + 1.
   bseg_index = sy-tabix.
   while sy-subrc = 0.
     bseg_index = bseg_index + 1.
     bseg_reads = bseg_reads + 1.
     read table bseg_tab
          into  bseg_lin
          index bseg_index.
     if bseg_lin-bukrs <> bkpf_lin-bukrs or
        bseg_lin-belnr <> bkpf_lin-belnr or
        bseg_lin-gjahr <> bkpf_lin-gjahr.
       sy-subrc = 99.
     else.
     endif.
   endwhile.
 endloop.

 get time field end_time.
 difference = end_time - start_time.

 write: /001 'Time for indexed loop:', difference,
        /005 'Number of BKPF reads:', bkpf_reads,
        /005 'Number of BSEG reads:', bseg_reads.
 skip 1.

endform.                    " indexed_loop

*&---------------------------------------------------------------------*
*&      Form  parallel_cursor
*&---------------------------------------------------------------------*
*       text
*----------------------------------------------------------------------*
FORM parallel_cursor.

 DATA: bkpf_index LIKE sy-tabix,
       bseg_index LIKE sy-tabix.

 CLEAR: bkpf_reads,
        bseg_reads.

 GET TIME FIELD start_time.

 SORT: bkpf_tab BY bukrs belnr gjahr,
       bseg_tab BY bukrs belnr gjahr.

 bseg_index = 1.
 LOOP AT bkpf_tab
   INTO bkpf_lin.
   bkpf_reads = bkpf_reads + 1.
   LOOP AT bseg_tab INTO bseg_lin FROM bseg_index.
     IF bseg_lin-bukrs <> bkpf_lin-bukrs OR
        bseg_lin-belnr <> bkpf_lin-belnr OR
        bseg_lin-gjahr <> bkpf_lin-gjahr.
       bseg_index = sy-tabix.
       EXIT.
     ELSE.
       bseg_reads = bseg_reads + 1.
     ENDIF.
   ENDLOOP.
 ENDLOOP.

 GET TIME FIELD end_time.
 difference = end_time - start_time.

 WRITE: /001 'Time for parallel cursor  :', difference,
        /005 'Number of BKPF reads  :', bkpf_reads,
        /005 'Number of BSEG reads  :', bseg_reads.

ENDFORM.                    " parallel_cursor