在SAP开发项目，这两个概念开发人员基本都知道。但是没有去对比其性能区别。
首先，我们普及一下原理。
Workarea: 相当于在内存堆栈里面开辟了一块内存。
Field Symbols:相当于存了内存的地址。所以实际上是不占用内存的。相当于C语言的已经被解除引用的指针。
所以我们进行一项性能测试：
1.对比单纯Loop的性能，不进行数据修改。
REPORT zloop_performance_test.
DATA : v_start_time TYPE timestampl,
v_end_time TYPE timestampl,
v_diff_w TYPE p DECIMALS 5.
TYPES : BEGIN OF ty_vbrp,
vbeln TYPE vbrp-vbeln,
END OF ty_vbrp.
DATA : lt_vbrp TYPE STANDARD TABLE OF ty_vbrp,
ls_vbrp TYPE ty_vbrp.
FIELD-SYMBOLS : TYPE ty_vbrp.
PARAMETERS p_count TYPE n LENGTH 6.
* Start of selection
START-OF-SELECTION.
* Get billing item data
SELECT vbeln
FROM vbap
INTO TABLE lt_vbrp
UP TO p_count ROWS.
END-OF-SELECTION.
* 1 . Loop with work area
GET TIME STAMP FIELD v_start_time.
LOOP AT lt_vbrp INTO ls_vbrp.
ENDLOOP.
GET TIME STAMP FIELD v_end_time.
v_diff_w = v_end_time - v_start_time.
WRITE :/(30) 'Loop with Work area', v_diff_w.
CLEAR : v_start_time, v_end_time, v_end_time.
* 2 . Loop with work area inline declaration
GET TIME STAMP FIELD v_start_time.
LOOP AT lt_vbrp INTO DATA(ls_vbrp_inline).
ENDLOOP.
GET TIME STAMP FIELD v_end_time.
v_diff_w = v_end_time - v_start_time.
WRITE:/(30) 'Loop with Inline Work area', v_diff_w.
CLEAR : v_start_time, v_end_time, v_end_time.
* 3 . Loop with Field symbol
GET TIME STAMP FIELD v_start_time.
LOOP AT lt_vbrp ASSIGNING .
ENDLOOP.
GET TIME STAMP FIELD v_end_time.
v_diff_w = v_end_time - v_start_time.
WRITE:/(30) 'Loop with Field Symbol', v_diff_w.
CLEAR : v_start_time, v_end_time, v_end_time.
* 4 . Loop with Field symbol inline declaration
GET TIME STAMP FIELD v_start_time.
LOOP AT lt_vbrp ASSIGNING FIELD-SYMBOL().
ENDLOOP.
GET TIME STAMP FIELD v_end_time.
v_diff_w = v_end_time - v_start_time.
WRITE:/(30) 'Loop with Inline Field Symbol', v_diff_w.
CLEAR : v_start_time, v_end_time, v_end_time. 
2.对比loop，并将数据修改如下面的代码：
REPORT zloop_performance_test.
DATA : v_start_time TYPE timestampl,
v_end_time TYPE timestampl,
v_diff_w TYPE p DECIMALS 5.
TYPES : BEGIN OF ty_vbrp,
vbeln TYPE vbrp-vbeln,
END OF ty_vbrp.
DATA : lt_vbrp TYPE STANDARD TABLE OF ty_vbrp,
ls_vbrp TYPE ty_vbrp.
FIELD-SYMBOLS : TYPE ty_vbrp.
PARAMETERS p_count TYPE n LENGTH 6.
* Start of selection
START-OF-SELECTION.
* Get billing item data
SELECT vbeln
FROM vbrp
INTO TABLE lt_vbrp
UP TO p_count ROWS.
END-OF-SELECTION.
* 1 . Loop with work area
GET TIME STAMP FIELD v_start_time.
LOOP AT lt_vbrp INTO ls_vbrp.
ls_vbrp-vbeln = 1.
MODIFY lt_vbrp FROM ls_vbrp.
ENDLOOP.
GET TIME STAMP FIELD v_end_time.
v_diff_w = v_end_time - v_start_time.
WRITE :/(30) 'Loop with Work area', v_diff_w.
CLEAR : v_start_time, v_end_time, v_end_time.
* 2 . Loop with work area inline declaration
GET TIME STAMP FIELD v_start_time.
LOOP AT lt_vbrp INTO DATA(ls_vbrp_inline).
ls_vbrp_inline-vbeln = 1.
MODIFY lt_vbrp FROM ls_vbrp_inline.
ENDLOOP.
GET TIME STAMP FIELD v_end_time.
v_diff_w = v_end_time - v_start_time.
WRITE:/(30) 'Loop with Inline Work area', v_diff_w.
CLEAR : v_start_time, v_end_time, v_end_time.
* 3 . Loop with Field symbol
GET TIME STAMP FIELD v_start_time.
LOOP AT lt_vbrp ASSIGNING .
-vbeln = 1.
ENDLOOP.
GET TIME STAMP FIELD v_end_time.
v_diff_w = v_end_time - v_start_time.
WRITE:/(30) 'Loop with Field Symbol', v_diff_w.
CLEAR : v_start_time, v_end_time, v_end_time.
* 4 . Loop with Field symbol inline declaration
GET TIME STAMP FIELD v_start_time.
LOOP AT lt_vbrp ASSIGNING FIELD-SYMBOL().
-vbeln = 1.
ENDLOOP.
GET TIME STAMP FIELD v_end_time.
v_diff_w = v_end_time - v_start_time.
WRITE:/(30) 'Loop with Inline Field Symbol', v_diff_w.
CLEAR : v_start_time, v_end_time, v_end_time. 
结论：
在两个测试结果中可以看出，loop到工作区实际上有一个内表到工作区的赋值过程。从工作区modify到内表也有一个赋值的过程。而在Field Symbol中则是直接修改了内表值。所以速度会比较快很多。
所以在能用Field Symbol的地方尽量使用Field Symbol。