What is a Field Symbols in SAP ABAP ?

Field symbols are similar to pointers in C language, field symbols does not  have any memory instead they will be pointing to a memory location.

The concept of field symbols is very important in order to increase the performance of SAP applications, but unethical use of field-symbols leads to application issues.

Field symbol name should always be within <>, example:.
Syntax for declaring a field symbol.

FIELD-SYMBOLS : <FIELD_SYMBOL> TYPE MARA-MATNR. "here MARA-MATNR is a variable type
FIELD-SYMBOLS : <FIELD_SYMBOL> TYPE  MARA. "here MARA is a structure
FIELD-SYMBOLS : <FIELD_SYMBOL> TYPE REF TO DATA . "here DATA is a reference type 

ASSIGNING and ASSIGN are the keywords which are used to assign a value to the field symbol.

Example of using field symbol as work area

In the below example we are going to use field symbol as work area.

REPORT ZSAPN_FIELDSYMBOLS.

DATA : IT_MARA TYPE TABLE OF MARA.
DATA : WA_MARA TYPE MARA.
FIELD-SYMBOLS : <FS_MARA> TYPE MARA.
SELECT * FROM MARA
  INTO TABLE IT_MARA UP TO 50 ROWS.

LOOP AT IT_MARA ASSIGNING <FS_MARA>.
  IF <FS_MARA> IS  ASSIGNED.
    WRITE :/ <FS_MARA>-MATNR, <FS_MARA>-MTART, <FS_MARA>-MEINS.
  ENDIF.
ENDLOOP.

Check weather the field symbol is assigned or not before processing field symbol, it is very important to check field symbol, if it is not assigned it will get a run-time error, use the blow syntax to check field symbol assignment.

  IF <FS_MARA> IS  ASSIGNED.
**process field symbol
  ENDIF.

Increase performance using field symbols in SAP ABAP

I belive there are always performance differences between reference variables(work area) and memory pointing(field symbols), here I wants to add some proof for that.

Below you will find two programs one with work area and another one with field symbols, execute them and see time difference at the end of the program( will display at the bottom of output), you will find difference.

program1

REPORT ZSAPN_FIELDSYMBOLS1.
DATA : IT_MARA TYPE TABLE OF MARA. "internal table
FIELD-SYMBOLS : <FS_MARA> TYPE MARA. "field symbols
DATA : LV_TIME_START TYPE TIMESTAMPL. "time at start of loop
DATA : LV_TIME_END TYPE TIMESTAMPL. "time at the end of loop
DATA LV_TIME_TAKEN TYPE P DECIMALS 8. "time difference

SELECT * FROM MARA "select data from MARA
  INTO TABLE IT_MARA UP TO 20000 ROWS.

GET TIME STAMP FIELD LV_TIME_START. "get time at start of loop
LOOP AT IT_MARA ASSIGNING <FS_MARA>. "loop start
  WRITE :/ <FS_MARA>-MATNR, <FS_MARA>-MTART, <FS_MARA>-MEINS.
ENDLOOP.

GET TIME STAMP FIELD LV_TIME_END. "get time at the end of loop
LV_TIME_TAKEN = LV_TIME_END - LV_TIME_START. "time taken
WRITE:/ 'Time taken: ', LV_TIME_TAKEN. "display time taken

Program2

REPORT ZSAPN_FIELDSYMBOLS2.
DATA : IT_MARA TYPE TABLE OF MARA. "internal table
DATA : WA_MARA TYPE MARA. "work area
DATA : LV_TIME_START TYPE TIMESTAMPL. "time at start of loop
DATA : LV_TIME_END TYPE TIMESTAMPL. "time at the end of loop
DATA LV_TIME_TAKEN TYPE P DECIMALS 8. "time difference

SELECT * FROM MARA "select data from MARA
  INTO TABLE IT_MARA UP TO 20000 ROWS.

GET TIME STAMP FIELD LV_TIME_START. "get time at start of loop
LOOP AT IT_MARA INTO WA_MARA.
  WRITE :/ WA_MARA-MATNR, WA_MARA-MTART, WA_MARA-MEINS.
ENDLOOP.

GET TIME STAMP FIELD LV_TIME_END. "get time at the end of loop
LV_TIME_TAKEN = LV_TIME_END - LV_TIME_START. "time taken
WRITE:/ 'Time taken: ', LV_TIME_TAKEN. "display time taken

See the time take to process loop in both programs.