You would get insight from Performance perspective when not using the Keys in SELECT with FOR ALL ENTRIES. System would spend definitely more time when there mixture of Entries in the ITAB.
Once you have selected records from one table and need to get the relevant records from the dependent table, you generally use FOR ALL ENTRIES IN ITAB addition of the SELECT query. This would return you the entries which matched keys from your ITAB specified after FOR ALL ENTRIES.
Most of the time, people select the data in ITAB and use that ITAB to get the data from another table. Like, you select the Materials from Sales Order items VBAP and use FOR ALL ENTRIES to select description from MAKT for the selected Materials. At this point, many people just pass all the selected entries from VBAP as FOR ALL ENTRIES FROM LT_SO_ITEMS. This is usually a performance drainage.
While using the FOR ALL ENTRIES, system selects all the records which meets the where condition. Once the data is selected, it removes the duplicate entries. E.g. if you have 1000 entries in your LT_SO_ITEMS and you use it in FOR ALL ENTRIES, it would select the records from MAKT for all 1000 entries even though there are only few say 50 unique materials. After the data selection, DB removes the duplicate records and present you the description for 50 materials.
Whenever you need to use the FOR ALL ENTRIES, you must always consider getting unique keys first before doing the SELECT. It may appear unnecessary work, but believe me, it would save you lot of time. Refer to the statistics at end of this post to figure out the performance improvement.
To get the unique keys:
- Declare a key table type
- Declare a ITAB_KEYS with this type
- LOOP AT main ITAB and append entries in the ITAB_KEYS
- SORT and DELETE adjacent duplicates
There would be other ways to achieve the Keys – Collect the table, READ the table entries before appending in it.
Check out the code lines and the numbers to see the performance improvement achieved when you use Unique Keys. In the code lines, there are 3 different approach to select the data.
- Using the Mix keys. You selected the records, you used it directly in FOR ALL ENTRIES
- Getting the Unique keys by doing the DELETE adjacent duplicates and then use in FOR ALL ENTRIES
- Getting the Unique keys by READ and then use in FOR ALL ENTRIES
REPORT Z_PERF_FOR_ALL_ENTRIES. TYPES: BEGIN OF lty_matnr, matnr TYPE mara-matnr, END OF lty_matnr. DATA: lt_matnr TYPE STANDARD TABLE OF lty_matnr. DATA: lt_makt TYPE STANDARD TABLE OF makt. DATA: lv_sta_time TYPE timestampl, lv_end_time TYPE timestampl, lv_diff_w TYPE p DECIMALS 5, lv_diff_f LIKE lv_diff_w, lv_save LIKE lv_diff_w. DATA: lt_mix_matnrs TYPE STANDARD TABLE OF lty_matnr. DATA: lwa_mix_matnrs LIKE LINE OF lt_mix_matnrs. DATA: lt_unique_matnrs TYPE STANDARD TABLE OF lty_matnr. DATA: lwa_unique_matnrs LIKE LINE OF lt_unique_matnrs. * Prepare data SELECT matnr INTO TABLE lt_matnr FROM mara UP TO 10 ROWS. " Change the number to get different numbers DO 1000 TIMES. APPEND LINES OF lt_matnr TO lt_mix_matnrs. ENDDO. *------- * 1. Mix keys *------- GET TIME STAMP FIELD lv_sta_time. SELECT * FROM makt INTO TABLE lt_makt FOR ALL ENTRIES IN lt_mix_matnrs WHERE matnr = lt_mix_matnrs-matnr. GET TIME STAMP FIELD lv_end_time. lv_diff_w = lv_end_time - lv_sta_time. WRITE: /(30) 'Mix Keys', lv_diff_w. *------- * 2. Unique Keys - DELETE *------- GET TIME STAMP FIELD lv_sta_time. LOOP AT lt_mix_matnrs INTO lwa_mix_matnrs. lwa_unique_matnrs-matnr = lwa_mix_matnrs-matnr. APPEND lwa_mix_matnrs TO lt_unique_matnrs. ENDLOOP. SORT lt_unique_matnrs BY matnr. DELETE ADJACENT DUPLICATES FROM lt_unique_matnrs COMPARING matnr. SELECT * FROM makt INTO TABLE lt_makt FOR ALL ENTRIES IN lt_unique_matnrs WHERE matnr = lt_unique_matnrs-matnr. GET TIME STAMP FIELD lv_end_time. lv_diff_f = lv_end_time - lv_sta_time. WRITE: /(30) 'Uniqe Keys - delete', lv_diff_f. *------- * 3. Unique Keys - READ *------- CLEAR: lt_unique_matnrs. GET TIME STAMP FIELD lv_sta_time. SORT lt_mix_matnrs BY matnr. LOOP AT lt_mix_matnrs INTO lwa_mix_matnrs. READ TABLE lt_unique_matnrs TRANSPORTING NO FIELDS WITH KEY matnr = lwa_mix_matnrs-matnr BINARY SEARCH. IF sy-subrc NE 0. lwa_unique_matnrs-matnr = lwa_mix_matnrs-matnr. APPEND lwa_unique_matnrs TO lt_unique_matnrs. ENDIF. ENDLOOP. SELECT * FROM makt INTO TABLE lt_makt FOR ALL ENTRIES IN lt_unique_matnrs WHERE matnr = lt_unique_matnrs-matnr. GET TIME STAMP FIELD lv_end_time. lv_diff_f = lv_end_time - lv_sta_time. WRITE: /(30) 'Uniqe Keys - Read', lv_diff_f.
I ran the report for different number of records. The numbers are like this:
If we make time takes by mix keys as 100%, the time taken by unique keys would look like this:
And on graph ..
More Performance Tuning tips in ABAP
Check out the other threads to learn more Performance Tuning in ABAP:
- Parallel Cursor – To speed up performance of Nested LOOP
- Parallel Cursor – 2: without using READ
- Use of Field-symbols vs Work area
- Measure the Performance between Break-Points using SE30
- READ-ONLY attribute vs GETTER methods
- Performance of ITAB Copy
- Performance of Using Keys in SELECT with FOR ALL ENTRIES
- ABAP Internal Table Performance for STANDARD, SORTED and HASHED Table
- ABAP Internal Table Secondary Key Performance comparison
- ABAP build a table with Unique Keys – Performance Comparison
- ABAP Parallel Cursor – Things to Remember