Monday, March 31, 2008



Sorted Tables, Hashed Tables and Key access to internal tables were introduced after release 4.0. Prior to this internal tables were declared as follows.

All internal tables had a header line. The syntax was as follows.

Data: BEGIN of <itab> occurs <n>,

END of <itab>

The number <n> in the occurs had the same meaning as the initial size as in release 4.0 onwards.


To fill a standard internal table without creating duplicate entries and add up the Packed, Integer, and Floating Point fields at the same time, use:


COLLECT uses a hash algorithm and is independent of the number of entries in the table (it does not need to maintain a table index). Therefore COLLECT is faster than READ BINARY combined with INSERT. If you need the final data sorted, the table should be sorted after all data has been collected.

Caution: When you fill an internal table, do not use COLLECT in combination with any other table filling statements (APPEND, INSERT, MODIFY, SELECT * INTO TABLE, SELECT * APPENDING TABLE). If you mix COLLECT with other statements, COLLECT cannot use it's hash algorithm and resorts to a normal linear search which is considerably slower.

Copying an internal table
Internal tables can be copied by MOVE like any other data object. If an internal table (itab) has a header line, the table can be accessed by itab[].

ITAB2[] = ITAB1[].

Processing tables only once. Your program should be structured so that your internal tables are processed only once, if possible. Seek to combine operations on an internal table so that they can all be processed in the one loop. The only situation where this may not be possible is where subsequent processing depends on the whole table having been processed previously (if the operations are linearly dependent they can still be done within the same loop).

READ instead of LOOP. If you only wish to retrieve a certain record from a table it is much more efficient (especially if the table is sorted) to use a READ WITH KEY or a READ INDEX on the table than looping until the record is found.

Sorted and Hashed tables: Following on from above, if you wish to reference a table several times within a program it is good practice to keep it sorted or to use a hashed table. Reads from normal unsorted internal tables have an order N time dependency (where N is the number of records), whereas sorted tables read with the command BINARY-SEARCH have a logarithmic time dependency (Log N), and hashed tables have a constant time dependency regardless of the number of records.
LOOP AT … WHERE. If you only wish to process certain entries of your internal table, use the LOOP AT.. WHERE command instead of a loop and then an IF or a CHECK statement.

In loops, only perform operations as necessary. If you are to perform an action on a field, remember to only perform the action when the value of the field changes and store the result. This means performing the operation within the ON CHANGE OF and AT NEW or AT END statements. WARNING Be aware that when using the AT statements, the order of the fields in your table or field-group is extremely important, if a field ‘higher up’ (further to the left) changes it will also trigger the AT statement on any field ‘lower down’ (further to the right).

APPEND LINES OF. When combining tables of the same structure, instead of looping through one table and appending the records to the other table, use this command that transfers the whole table in one operation. This is particularly useful if you require the retrieval of the same data from two different database tables, in which case you can use two SELECT INTO and then an APPEND LINES OF to combine your data.
Syntax: APPEND LINES OF itab1 [FROM idx1] [TO idx2] TO itab2.

READ TABLE … TRANSPORTING NO FIELDS. This statement is the fastest way to find a particular record within an internal table (when combined with the most efficient searching technique for that table, such as a hashed table or a read using a binary search). This command is used to set the sy-tabix field and the sy-subrc field to indicate position of the record and if found successfully, these fields can then be used to insert new records or to continue processing knowing the record exists.

See Also:


Related Interview Questions

1) COLLECT can be used to add up the Packed, Integer, and Floating Point fields at the same time.                     TRUE
2) COLLECT is faster than READ BINARY combined with INSERT                                                                                 TRUE
3) Mixing collect with other statements
4) COLLECT uses a hash algorithm                                                                                                                                 TRUE
5) ITAB2[] = ITAB1[]. will copy ITAB2[] into ITAB1[]                                                                                                            FALSE

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Always specify your conditions in the Where-clause instead of checking them yourself with check statements. The database system can then use an index (if possible) and the network load is considerably less.

For all frequently used Select statements, try to use an index. You always use an index if you specify (a generic part of) the index fields concatenated with logical Ands in the Select statement's Where clause. Note that complex Where clauses are poison for the statement optimizer in any database system.

If there exists at least one row of a database table or view with a certain condition, use the Select Single statement instead of a Select-Endselect-loop. Select Single requires one communication with the database system, whereas Select-Endselect needs two.

It is always faster to use the Into Table version of a Select statement than to use Append statements.

To read data from several logically connected tables use a join instead of nested Select statements. Network load is considerably less.

If you want to find the maximum, minimum, sum and average value or the count of a database column, use a select list with aggregate functions instead of computing the aggregates yourself. Network load is considerably less.

If you process your data only once, use a Select-Endselect-loop instead of collecting data in an internal table with Select Into Table. Internal table handling takes up much more space.

Use a select list or a view instead of Select * , if you are only interested in specific columns of the table. Network load is considerably less.

For all frequently used, read-only tables, try to use SAP buffering. Network load is considerably less.

Whenever possible, use array operations instead of single-row operations to modify your database tables. Frequent communication between the application program and database system produces considerable overhead.

Whenever possible, use column updates instead of single-row updates to update your database tables. Network load is considerably less.

Instead of using nested Select loops or FOR ALL ENTRIES it is often possible to use subqueries. Network load is considerably less.

Use the special operators CO, CA, CS, instead of programming the operations yourself. If ABAP/4 statements are executed per character on long strings, CPU consumption can rise substantially.

Some function modules for string manipulation have become obsolete and should be replaced by ABAP/4 statements or functions: STRING_CONCATENATE... -> CONCATENATE, STRING_SPLIT... -> SPLIT, STRING_LENGTH -> strlen(), STRING_CENTER -> WRITE...TO...CENTERED, STRING_MOVE_RIGHT -> WRITE...TO...RIGHT-JUSTIFIED

Use the CONCATENATE statement instead of programming a string concatenation of your own.

If you want to delete the leading spaces in a string, use the ABAP/4 statement SHIFT...LEFT DELETING LEADING... .Other constructions (with CN and SHIFT...BY SY-FDPOS PLACES, with CONDENSE if possible, with CN and ASSIGN CLA+SY-FDPOS(LEN) ...) are not as fast. In any case, avoid using SHIFT inside a WHILE-loop!

Use the SPLIT statement instead of programming a string split yourself.

Use the strlen( ) function to restrict the DO loop to the relevant part of the field, e.g. when determinating a check-sum.

Use "CLEAR f WITH val" whenever you want to initialize a field with a value different from the field's type-specific initial value.

Try to keep the table ordered and use binary search or used a table of type SORTED TABLE. If TAB has n entries, linear search runs in O( n ) time, whereas binary search takes only O( log2( n ) ).

A dynamic key access is slower than a static one, since the key specification must be evaluated at runtime. However, for large tables the costs are dominated by number of comparison needed to locate the entry.

If you need to access an internal table with different keys repeatedly, keep your own secondary indices.With a secondary index, you can replace a linear search with a binary search plus an index access.

LOOP ... WHERE is faster than LOOP/CHECK because LOOP ... WHERE evaluates the specified condition internally. As with any logical expressions, the performance is better if the operands of a comparison share a common type. The performance can be further enhanced if LOOP ... WHERE is combined with FROM i1 and/or TO i2, if possible.

Always use Pretty Printer and Extended Program Check before releasing the code. Do not leave unused code in the program. Comment the code thoroughly. Align the comments and the Code. Follow the SAP Standards and SAP Best Practices guidelines. It’s a good practice to take a dump of the code on your local drive.

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