APPEND vs COLLECT statements
1. Avoid unnecessary assignments to the header line when using internal tables with a header line. Whenever possible, use statements that have an explicit work area.
For example: "APPEND wa TO itab." is approximately twice as fast as "itab = wa. APPEND itab.". The same applies to COLLECT and INSERT.
2. In contrast to COLLECT, APPEND does not check whether an entry with the same key exists. Therefore, it is considerably faster than COLLECT. If the COLLECT logic is not needed or lines with an identical default key cannot occur in a particular situation, you should always use APPEND instead of COLLECT.
3. COLLECT allows you to create a unique or summarized dataset, and you should only use it when this is necessary. If neither of these characteristics are required, or where the nature of the table in the application means that it is impossible for duplicate entries to occur, you should use INSERT [wa INTO] TABLE itab instead of COLLECT. If you do need the table to be unique or summarized, COLLECT is the most efficient way to achieve it.
4. If you use COLLECT with a work area, the work area must be compatible with the line type of the internal table.