Difrrence between fld grps and fld symbols

Question

Hi,
what are the differences between field groups and field symbols??

Answer 1

HI
<b>
Field groups:</b>
http://help.sap.com/saphelp_46c/helpdata/EN/d2/cb43e6455611d189710000e8322d00/content.htm
<b>A Note on the Use of Field Groups...</b>
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).
Field-Groups Sample Code
*& Report  ZSPFLI                                                      *
REPORT  ZSPFLI  LINE-SIZE 132 LINE-COUNT 65(3)
                                             NO STANDARD PAGE HEADING.
TABLES:SPFLI,SCARR, SFLIGHT, SBOOK.
SELECT-OPTIONS: MYCARRID FOR SPFLI-CARRID.
FIELD-GROUPS: HEADER, SPFLI_FG, SFLIGHT_FG, SBOOK_FG.
INSERT:
        SPFLI-CARRID
        SPFLI-CONNID
        SFLIGHT-FLDATE
        SBOOK-BOOKID
       INTO HEADER,
        SPFLI-CARRID
        SPFLI-CONNID
        SPFLI-CITYFROM
        SPFLI-AIRPFROM
        SPFLI-CITYTO
        SPFLI-AIRPTO
        SPFLI-DEPTIME
        SCARR-CARRNAME
      INTO SPFLI_FG,
        SFLIGHT-FLDATE
        SFLIGHT-SEATSMAX
        SFLIGHT-SEATSOCC
        SFLIGHT-PRICE
      INTO SFLIGHT_FG,
        SBOOK-BOOKID
        SBOOK-CUSTOMID
        SBOOK-CUSTTYPE
        SBOOK-SMOKER
       INTO SBOOK_FG.
SELECT * FROM SPFLI WHERE CARRID IN MYCARRID.
  SELECT SINGLE * FROM SCARR WHERE CARRID = SPFLI-CARRID.
  EXTRACT SPFLI_FG.
  SELECT * FROM SFLIGHT
   WHERE CARRID = SPFLI-CARRID AND  CONNID = SPFLI-CONNID.
    EXTRACT SFLIGHT_FG.
    SELECT * FROM SBOOK
           WHERE CARRID = SFLIGHT-CARRID AND
           CONNID = SFLIGHT-CONNID AND FLDATE = SFLIGHT-FLDATE.
      EXTRACT SBOOK_FG.
      CLEAR SBOOK.
    ENDSELECT.
    CLEAR SFLIGHT.
  ENDSELECT.
  CLEAR SPFLI.
ENDSELECT.
SORT.
LOOP.
  AT SPFLI_FG.
    FORMAT COLOR COL_HEADING.
    WRITE: / SCARR-CARRNAME,
             SPFLI-CONNID, SPFLI-CITYFROM,
             SPFLI-AIRPFROM, SPFLI-CITYTO, SPFLI-AIRPTO, SPFLI-DEPTIME.
    FORMAT COLOR OFF.
  ENDAT.
  AT SFLIGHT_FG.
    WRITE: /15 SFLIGHT-FLDATE, SFLIGHT-PRICE, SFLIGHT-SEATSMAX,
               SFLIGHT-SEATSOCC.
  ENDAT.
  AT SBOOK_FG.
    WRITE: /30 SBOOK-BOOKID, SBOOK-CUSTOMID,
                 SBOOK-CUSTTYPE, SBOOK-SMOKER.
  ENDAT.
ENDLOOP.
*&      END OF REPORT                                                  *
<i><b>
Field Symbols </b></i> Field symbols are placeholders or symbolic names for other fields. They do not physically reserve space for a field, but point to its contents. A field symbol cam point to any data object. The data object to which a field symbol points is assigned to it after it has been declared in the program.
Whenever you address a field symbol in a program, you are addressing the field that is assigned to the field symbol. After successful assignment, there is no difference in ABAP whether you reference the field symbol or the field itself. You must assign a field to each field symbol before you can address the latter in programs.
Field symbols are similar to dereferenced pointers in C (that is, pointers to which the content operator * is applied). However, the only real equivalent of pointers in ABAP, that is, variables that contain a memory address (reference) and that can be used without the contents operator, are reference variables in ABAP Objects. (For more information, see Data References).
All operations programmed with field symbols are applied to the field assigned to it. For example, a MOVE statement between two field symbols moves the contents of the field assigned to the first field symbol to the field assigned to the second field symbol. The field symbols themselves point to the same fields after the MOVE statement as they did before.
You can create field symbols either without or with type specifications. If you do not specify a type, the field symbol inherits all of the technical attributes of the field assigned to it. If you do specify a type, the system checks the compatibility of the field symbol and the field you are assigning to it during the ASSIGN statement.
Field symbols provide greater flexibility when you address data objects:
•     If you want to process sections of fields, you can specify the offset and length of the field dynamically.
•     You can assign one field symbol to another, which allows you to address parts of fields.
•     Assignments to field symbols may extend beyond field boundaries. This allows you to address regular sequences of fields in memory efficiently.
•     You can also force a field symbol to take different technical attributes from those of the field assigned to it.
The flexibility of field symbols provides elegant solutions to certain problems. On the other hand, it does mean that errors can easily occur. Since fields are not assigned to field symbols until runtime, the effectiveness of syntax and security checks is very limited for operations involving field symbols. This can lead to runtime errors or incorrect data assignments.
While runtime errors indicate an obvious problem, incorrect data assignments are dangerous because they can be very difficult to detect. For this reason, you should only use field symbols if you cannot achieve the same result using other ABAP statements.
For example, you may want to process part of a string where the offset and length depend on the contents of the field. You could use field symbols in this case. However, since the MOVE statement also supports variable offset and length specifications, you should use it instead. The MOVE statement (with your own auxiliary variables if required) is much safer than using field symbols, since it cannot address memory beyond the boundary of a field. However, field symbols may improve performance in some cases.
Defining Field Symbols
Assigning Data Objects to Field Symbols
<b> Declaring Field Symbols </b>
To declare a field symbol, use the statement
FIELD-SYMBOLS <FS> [<type> STRUCTURE  DEFAULT <wa>].
For field symbols, the angle brackets are part of the syntax. They identify field symbols in the program code.
If you do not specify any additions, the field symbol <FS> can have data objects of any type assigned to it. When you assign a data object, the field symbol inherits its technical attributes. The data type of the assigned data object becomes the actual data type of the field symbol.
Note: it is possible to assign reference variables and structured data objects to untyped field symbols. However, the static field symbol is only a pointer to the field in memory, and does not have the complex type attributes of a reference or structured field until runtime. You can only use the field symbol to address the whole field (for example, in a MOVE statement). Specific statements such as CREATE OBJECT <FS> or LOOP AT <FS> are not possible.
Regards
Ravish Garg<b><i>
*reward if helpful</i></b>