varchar2設置長度與實際大小

以前有學生問過，varchar2(4k)與varchar2(1)裏面存同樣的數據會怎麼樣。

我喜歡直接用測試結果說話，現在把實驗補上

CREATE TABLE vc1(c1 VARCHAR2(1));
CREATE TABLE vc4k(c1 VARCHAR2(4000));

分別加入1W行數據進去

SQL> insert into vc1 select * from vc4k;
1000000 rows inserted

SQL> select s.BYTES/1024/1024 from user_segments s where s.segment_name in(upper('vc4k'),upper('vc1'));
S.BYTES/1024/1024
-----------------
               13
               13
2 rows selected

SQL> select count(*) from vc1 union all select count(*) from vc4k;
  COUNT(*)
----------
   1000000
   1000000
2 rows selected

SQL> select c.TABLE_NAME,c.COLUMN_NAME,c.DATA_TYPE,c.DATA_LENGTH from user_tab_columns c where c.TABLE_NAME in(upper('vc4k'),upper('vc1'));
TABLE_NAME COLUMN_NAM DATA_TYPE  DATA_LENGTH
---------- ---------- ---------- -----------
VC1        C1         VARCHAR2             1
VC4K       C1         VARCHAR2          4000
2 rows selected

SQL> 

segment一樣

各自增加1W行再看。

SQL> insert into vc1 select * from vc4k;
1000000 rows inserted

SQL> insert into vc4k select * from vc4k;
1000000 rows inserted

SQL> select s.BYTES/1024/1024 from user_segments s where s.segment_name in(upper('vc4k'),upper('vc1'));
S.BYTES/1024/1024
-----------------
               25
               25
2 rows selected

SQL> 

仍然一樣