Performance between CLOB and VARCHAR2

Question

I would like to know if there is any really performance issues between CLOB and VARCHAR2 data types?
In particular, why would it not be better to declare all large text items as CLOB rather than VARCHAR2(4000) in a table? For example, if I am going to store about a page of text, in a table, why not standardize of CLOB? Only use VARCHAR2 for small text items.

Answer 1

I doubt that there would be much, if any, performance difference between a CLOB and a VARCHAR2. By default, Oracle will store CLOBS directly in the table if they are less than about 4000 bytes, so the effect would be the same as a VARCHAR2(4000).
The advantage of VARCHAR2(4000) over a CLOB is that you document and enforce the maximum size of the field. If you know that no test item will exceed 4000 bytes, then I would store it as a VARCHAR2, because if they can store more, someone will.
A possible disadvantage of using CLOBS instead of VARCHAR2(4000) is that when you declare a column as a CLOB, Oracle creates the two lob segments whether or not they are needed to actually store data. So, depending on how many VARCHAR2(4000) columns you change to CLOBS without needing to store more than 4000 bytes, you can potentially waste a significant amount of space.
SQL> CREATE TABLE t_clob (id NUMBER, descr CLOB);
Table created.
SQL> SELECT segment_name, index_name
  2  FROM dba_lobs
  3  WHERE table_name = 'T_CLOB';
SEGMENT_NAME                   INDEX_NAME
SYS_LOB0000136329C00002$$      SYS_IL0000136329C00002$$
SQL> SELECT segment_name, segment_type, blocks
  2  FROM dba_segments
  3  WHERE segment_name in ('SYS_LOB0000136329C00002$$','SYS_IL0000136329C00002$$')
SEGMENT_NAME                   SEGMENT_TYPE           BLOCKS
SYS_IL0000136329C00002$$       LOBINDEX                   64
SYS_LOB0000136329C00002$$      LOBSEGMENT                 64HTH
John