Field symbols are similiar to pointers in C. It stores the memory address of a data element(A record). BY using the statement ASSIGN<> to you can assigning a structure dynamically. In procedural programming assigning field symols of type any is permitted(upto 4.6C).
Because field-groups write their data to paging space (rather than storing it in memory), they are appropriate only for processing lists with lots (like 50,000 or more) of records. If you expect your programs to be handling tens of thousands of records, you should:
analyze the expected size of your lists. For instance, if your system has 512M of main memory, you may decide that you don't want any report to use more than 15M of memory for its lists. In that program, you may have a list:
begin of mylist occurs XXX,
dat1(100) type c,
dat2(50) type c,
dat3(10) type c,
end of list.
Then each record takes up approximately 160 bytes; so every 6 records take up approximately 1K. For this list structure, it would take about 90,000 records to use up 15M RAM.
decide the maximum amount of memory you want your program to use
decide whether to use field-groups or something else (like internal tables). If you expect the size of your list to be greater than the amount of memory you want your program to use, then use field-groups (actually, if you use internal tables, and the number of records exceeds the number of records in your OCCURS statement, the system just writes those extra records to the paging space. So is there really any difference between just using an internal table with an OCCURS 0 statement-- which would write the entire table to paging space-- and using field-groups? According to Gareth M. de Bruyn and Robert Lyfareff in Introduction to ABAP/4 Programming for SAP, field-groups are stored more efficiently, and have better performance. They recommend field-groups for lists of 100,000 or more records).
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